Reimmunization after allogeneic bone marrow transplantation

1

/

Reimmunization Transplantation

REVIEW

After Allogeneic Bone Marrow

Jyoti Somani, MD, Richard A. Larson, MD, Chicago, l/his

Allogeneic bone marrow transplant patients are severely immunocompromised during the immediate poattran@ant period, and the risk for common and opportunlatic infections may persist for many menths. The role of reimmwnizatlon for these patients, however, remains unsettled. We briefly review current concepts regarding the recapitulation of immunity from the totlpotential hematopoietlc stem cells in the donor marrow. The fact thet various components of the new immune system mature at different rates can have clinical coneequences with regard to specific itrfectianrs. Most previously immunized patients become antibody seroneaatlve within a few months after allogeneic marrow transplantation. Adoptive tranefer of apecific antibody-producing cells from the donor to the recipient has been demonstrated in small clinical trials, and is augmented when both donor and recipient are vaccinated. Passive transfer of immunity is more easily achieved to recall ant&ma than to neoantigens. Primary immunization requires prolonged a’ntigenic stimulation and mature T-ceil function or help from natura’l klllkaacells. Most healthy patients generate adequate ant&n& t4ters to vaccinations that are given 12 months after transplantation, but the presence of chronic graft-versus-host disearoe can diminish the response. Currently available vaccines have been evaluated in marrow transplant patients. Protein antigens such BStetanus and diphtheria toxoids are more immunogenic than poiysaccharide antigens such as,pneumococcal vaccine. The new polysaccharide~protein conjugate vaccines, such as the lfcsmeph&s infiuenzae type b vaccine, also appear more immunegenic. Inactivated poliovirus vacciine has been used successfully. Relatively few data are available about hepatitis B or influenza vale&es.

From the Section of Hematology/Oncology, Department of Medicme, The University of Chicago, Chicago, Illinois. Requests for reprints should be addressed to Richard A. Larson, MD, University of Chicago Medical Center, 5841 South Maryland Avenue, MC2115, Chicago, IL 60637. Manuscript submitted August 2, 1994 and accepted In revised form December 16, 1994. I

The literature supports the use of standard vaccines in allogeneic bone marrow transplant patients. However, more data on the optimal methods and timing of immunization are needed. We present guidelines for a reimmunization schedule.

B

one marrow transplantation

(EMT) is being performed to treat an expanding list of malignant and nonmalignant disorders. Posttransplant care is shared between specialized BMT centers and community physicians. Thus, internists need to become familiar with the management of these patients. BMT recipients remain immunocompromised and susceptible to a large variety of common and opportunistic infections for many months posttransplant. In addition to the use of antibiotics and hematopoietic growth factors, investigators are exploring the use of vaccines to protect BMT patients against common pathogens. The role of reimmunization following allogeneic bone marrow transplantation remains unsettled for a number of reasons. First, no large study has examined the posttransplant persistence of antibody titers generated by previous immunizations against diphtheria, pertussis, and tetanus (DPT); mumps, measles, and rubella (MMR); and polio. Arguably, these particular infections do not pose major risks to adult BMT patients. Even less attention has been paid, however, to posttransplant immunity to the far more common pneumococcal and hepatitis B pathogens. Second, the differences in immune recovery following T-cell-depleted versus nonmodified-marrow grafts-especially in the ability to produce specific responses to antigens-have not been well delineated. Third, graft-versus-host disease (GVHD) and its treatment both clearly hinder immune recovery, but any resultant limitations on successful vaccination remain unexplored. Last, but equally important, the benefits and potential risks of administering any of the available vaccines to BMT patients have not been firmly established by clinical trials. This paper reviews the literature on immunization after allogeneic BMT from the perspective of what is known about immune recovery. Techniques to enhance the response to vaccines are highlighted, particularly those that elicit optimal effects. We cite clinical recommendations and directions for further research. April 1995 The American Journal of Medicine@ Volume 98

389

REIMMUNIZATION FOLLOWING BMT/SOMANI

AND LARSON

lMMUNOLOGlC RECOVERY AFTER ALLOGENEIC BMT

cell antibody responses that are T-cell-dependent are generated primarily by degradable protein antigens that lack repeating epitopes-eg, vaccines such as tetanus toxoid, diphtheria toxoid, poliomyelitis, or measles. This type of B-cell antibody response establishes memory, and is characterized by an enhanced, long-lasting, predominantly IgG anamnestic response to subsequent challenges with the same antigen. In contrast, T-cell-independent responses are elicited mainly by undegradable polysaccharide antigens with repeating epitopes (eg, pneumococcal polysaccharide). These responses are weaker, result in nonenhanced, short-lived IgM responses, and often do not establish long-term memory. Studies in vitro have demonstrated that the transplanted immune system is unable to generate specific IgG responses to neoantigens (ones never “seen” before) or to recall antigens (ones that have previously been “seen”) during the first 3 to 6 months.4J7 In addition to specific stimuli, normal B cells respond to nonspecific mitogens with activation of many clones. Many of these polyclonal activators, such as keyhole-limpet hemocyanin, pokeweed mitogen, and Staphylococcus a.urtws, are T-cell-dependent B-cell mitogens. In vitro observations have disclosed that immune responses to them are impaired after BMT.4111J”Js The deficiencies reflect a failure of B cells to secrete immunoglobulins and abnormal regulation by T cells,1”J8-z but apparently not defects in monocytes.g Within the first 100 days following transplantation of unmodified marrow, an inversion of the normal ratio of T-suppressor (CD8) to T-helper (CD4) cells occurs as a result of an absolute increase in T-suppressor and an absolute decrease in T-helper cells2” ‘5 This inverse T-cell ratio does not correlate with the type of graft or the early presence of GVHD, but appears to be inherent to the immune recovery process.1”Bti,26 Among patients with chronic GVHD the T-suppressor predominance persists for as loni as 3 years after BMT.26 T-helper activity remains defective and may be responsible for inhibiting immunoglobulin synthesis throughout this period.z7 Whether chronic GVHD contributes primarily or secondarily to this situation has not been established. Several studies have demonstrated that disordered B- and T-cell function often persists in chronic GVHD, resulting in continued immunodefi-

All BMT recipients are profoundly immunosuppressed for several months after transplantation, regardless of the type of graft (autologous, syngeneic, or allogeneic), the underlying disease, the conditioning and preparative regimens, and the prophylaxis or presence of acute GVHD.‘” Recipients who do not develop significant GVHD or other complications recover marrow function rapidly, resuming near-normal production of blood cells within 4 to 8 weeks. Reconstitution of a fully functioning immune system occurs more slowly. The pace of immunologic recovery in syngeneic and allogeneic marrow recipients is remarkably similar despite the absence of GVHD and immunosuppressive agents in the former. Primitive functions such as proliferation and phagocytic activity resume early, usually within the first 100 days. Antigen-presenting cells become active relatively early,7-9 although functionally distinct subclasses of antigen-presenting cells have various recovery times.7 Normal monocyte accessory function has been documented within 3 months and found to be unaffected by the presence of GVHD.” Antibodyand lectin-dependent cellular cytotoxicities and cytotoxic activity mediated by natural killer (NK) cells can all return to normal by 30 to 90 days posttransplant.‘@12 However, at least one study showed that approximately 20% of patients were still deficient in these functions beyond 1 year.lO This same study reported no significant association between recovery of cellular cytotoxic capability and the presence of GVHD, infection, or recurrent leukemia. Despite the early reappearance of primitive immune functions, many patients retain combined humoral and cellular immunologic deficiencies for as long as 1 to 2 years after transplantation. The presence of chronic GVHD can prolong this period still further.1:sJ4 It is now believed that the entire lymphocyte lineage must be recapitulated from the transplanted totipotential stem cells or from very early lymphoid precursor cells. The numbers of circulating B and T cells and absolute serum immunoglobulin (Ig) levels normalize 4 to 6 months after engraftment, but functional deficits of these component.9 persist longer.15zLfi Serum IgA concentrations remain low for prolonged periods of time. Serum IgE levels are often elevated in patients with chronic GVHD. The variable rates of recovery of specific subtypes of immunoglobulins and their significance are discussed later. Despite functioning antigen-presenting cells, immune responses entailing complicated B- and T-cell interactions are diminished during the first year af-

An immune response to skin-test antigens requires coordinated interaction between effector B and T cells. In one study, 332 marrow graft recipients and 241 healthy marrow donors underwent skin testing to evaluate their responses to neoantigens and recall antigens.2Y BMT patients’ responses to the

ter

engraftment

neoantigen keyhole-limpet hemocyanin were lower

390

April

1995

and

often

The American

longer.i311” Journal

In

general,

of Medicine@

Volume

B98

ciency.

19.27,28

REIMMUNKATION

than normal regardless of the interval after grafting. Their responses to the neoantigen dinitrochlorobenzene were equal to those observed in the normal donors at 3 months, but subsequently fell below normal for up to 2 years after the transplant. This fmding was interpreted as evidence of mosaicism with temporary persistence of host-cell functioning alongside infused mature donor cells. When the recall antigens Candida, mumps, streptokinasestreptodornase, and Trichophyton were administered intradermally at regular intervals, BMT patients responded less well than normal until 4 years after EMT. Overall, acute ad chronic GVHD were associated with anergy over the entire 3-year testing period. Patients with chronic GVHD were statistically more likely to be anergic at 2 years posttransplantation. Thus, delayed-type hypersensitivity, which requires effective interaction between B and T cells, was diminished for a prolonged period in BMT recipients who did not have GVHD, and often indefinitely in patients with GVHD. Although total serum immunoglobulin levels normalize within 3 to 6 months after BMT, deficiencies in specific immunoglobulin isotypes that predispose patients to particular infections can be long lasting.“” A marked deficiency in plasma-cell production of IgG2 and IgG4 isotypes along with a relative increase in IgGl and IgG3 isotypes have been documented both in vitro and in vivo.30,91 These deficiencies persist for more than 18 months. Interestingly, the pattern of isotype switching during recovery after BMT reproduces the steps of normal B-cell ontogeny.30J” Thus, normal adult serum levels of specific Ig isotypes occur in similar sequence-ie, IgM-+IgGl+IgG3+IgG2+IgG4+ &A---in infants and BMT patients. IgG2 plasma cells are the main responders to bacterial polysaccharide antigens, and the deficiency of this subclass leads to increased susceptibility to such infections as Streptococcus pneumoniae, Neisser-ia meningitidis, and Hemophilus in$Zuenxae.:33 For this reason, during the first several months posttransplant, patients often receive immunoglobulin infusions for prophylaxis. Patients with chronic GVHD have profound and persistent IgG2 deficiency and are particularly prone to these infections, especially late (6 to 24 months) after transplantation. The patterns of normal immune recovery after BMT have been described, but much remains to be learned about the functional recovery of the immune system, especially in the presence of GVHD or following transplantation with T-cell-depleted marrow. A better understanding of the patterns of infection in the posttransplant period has led to specific prophylactic treatments or (ie, cotrimoxazole aerosolized pentamidine for Prwumocystis catin.i%,

FOLLOWNG

BM’l/SOMANI

AND LARSON

acyclovir for herpes simplex virus, and penicillin for S pneumonias). In addition, supportive therapies such as intravenous immunoglobulin and hematopoietic growth factors have lowered mortality folIowing BMT.

IS THERE A NEED FOR REMWFMWWN AFTER BMT? Most pediatric centers reimmunize children empirically with the DPT, MMR, and polio vaccines 1 to 2 years after BMT because children and their siblings are frequently exposed to these infections. Moreover, the incidence of GVHD and the resultant immunodeficiency is much lower in the pediatric population, allowing early revaccination and a more dependably positive response. About half of adult BMT patients lose their seropositivlty to tetanus, MMR, and diphtheria when titers are followed serially over 1 to 2 years after transplantation.3p37 One study found that 10 of 15 patients with chronic GVHD had lost their titers to tetanus toxoid by 2 years, compared to only 1 of 11 healthy BMT survivors3” In this same study, 5 of 12 patients whose antidiphtheria titers were measured had none after 2 years, with no statistical difference in this finding between those with GVHD and healthy survivors. The results were similar in patients whose antimeasles titers were measured a decline after 2 years, but no more marked among those with chronic GVHD. Because individual patients’ titers were not measured sequentially, and the numbers in each group were quite small, it is difficult to draw solid conclusions from this study. I,jungman et, aly4 surveyed immunity to MMR viruses in 57 patients (median age 17 years) for as long as 2 years after allogeneic BMT. Among patients who were seropositive at the time of transplant, the seropositive rates 2 years later for MMR were 51%, 42%, and 76%. GVHD was not associated with an increased loss of seropositivity. Another study focused only on tetanus toxoid titers in 48 patients undergoing BMT, 37 of whom were seropositive at the time of transplantation.35 After 1 year, only 18 patients (49??) maintained titers to tetanus, and all who were not subsequently reimmunized with tetamrs toxoid were seronegative by 2 years. Again, GVHD did not influence the seroconversion rates. I&ngman et a$% studied long-term immunity to the inactivated poliovirus vaccine (II?‘) in 55 patients and reported that approximately half sustained at least a fourfold loss of antibody titers by 12 months after transplantation. As in earlier studies, there was no association between chronic GVHD and seroconversion. Although limited by small sample sizes, these studies support the need for reimmunization with standard vaccinations after BMT. The questions then become: How well will BMT recipients respond to April

1995

The American

Journal

of Medicine”

Volume

98

391

REIMMUNiZATlON

FOLLOWING BW/SOh’lANI

AND LARSON

cate the real degree of immune protection. In the study just discussed, it is uncertain whether the antibody synthesis was protective, in part because neither diphtheria nor tetanus are common in the postBMT setting. The major import of the experiment is IS THERE ADOPTIVE TRANSFER OF that it provides evidence for transfer of specific imANTiBODY RlWOMsE TO RECALL mune memory, since the recipients whose donors had ANTIGENS DURING BMT? received vaccines responded to reimmunization during the first 5 months post-BMT. The reason for a meaThe transfer of donor immunity has been documented in previously seronegative recipients who convert- surable response only to the tetanus vaccine is uned after receiving marrow from seropositive donors, clear, but may be attributable to the higher molecular but this immunity is not long lasting, and patients need weight of tetanus toxoid. Te&mus toxoid is also more to be reimmunized.34,36*38 The transient nature of the antigenic than diphtheria in healthy volunteers. Wimperis and colleagues30 transferred donor impassive transfer of donor antibody production is likely due to the loss over time of the transfused mature im- munity by vaccinating 9 donors and recipients with munocompetent donor cells, which occurs more tetanus toxoid and hepatitis B vaccine 1 week prior BMT. The antibody responses quickly than the recapitulation of new immune effec- to T-cell-depleted tors from immunologically naive primitive donor stem lasted as long as 3 months. In 16 other cases, only the donor was immunized. Antibody responses were cells. Several experiments have attempted to engineer transferred, but they were short-lived, and the recipthe adoptive transfer of specific antibody-producing cells from the donor to confer lasting immunity in the ients’ antibody titers fell to baseline by 8 weeks. Thus, recipient.39,41a42 Early studies with tetanus toxoid another way to stimulate production of protective IgG found that healthy human bone marrow cells respond antibody to recall antigens may be to vaccinate both donor and recipient prior to BMT, then further reimdifferently to primary and booster immunizations.~~ The passive transfer from donor to recipient. of anti- munize the recipient posttransplant in order to elicit body synthesis to a protein recall antigen occurred and expand the immune memory. even in the absence of Tcell help, whereas the transSubsequent experiments confn-med that vaccinatfer of antibody response to a primary or neoantigen ing the donor led to adoptive transfer of antibody revia BMT failed. The adoptive transfer of specific anti- sponses to recall antigen, and that vaccinating the recipient, t,oo, enhanced the response. In contrast, the body-producing cells has been successfully demonstrated for neoantigens, such as keyhole-limpet he- adoptive transfer of antibody response to a primary mocyanin, only after prolonged antigen stimulation antigen was achieved only when both donor and retogether with Tcell or NK-cell help.“gB41s42 cipient were immunized prior to transplantation.42 Saxon et a141studied the acquisition of post-BMT Thus, it was concluded that, “The immunization proantigen-specific humoral immune response and tocols required for the transfer of antibody responses memory. Ten donor/recipient pairs were followed. to priming antigen reflect the initial dependence of Seven donors received diphtheria/tetanus toxoid unprimed B cells on T-cell help and on prolonged priboosters prior to transplantation. In each case, the mary antigen stimulation.” Immunization of the retransplanted bone maTow contained B cells that pro- cipient prior to transplantation presumably results in duced IgG to tetanus and diphtheria toxoids, and the the activation of chemoradioresistant antigen-prerecipient showed a corresponding rise in both anti- senting cells in the host, whiah then continue to stimbody titRrs. Three of the 7 received a single diphtheulate the transplanted B cells, &wing a more intense ria/tetanus toxoid booster immunization 64 to 1.54 and persisting antibody response. Alternatively, the days posttransplant. Serum antitetanus antibody lev- antigen itself may persist in the host. These findings els increased 28 days after reimmunization, but serum need to be considered when attempts are undertaken antidiphtheria levels did not. In contrast, the 3 recip- to immunize recipients with such primary agents as ients whose donors were not immunized prior to Pseudomonas species, cytomegalovirus, or varicella transplantation failed to exhibit increases in either an- zoster virus. tibody after receiving a diphtheria/tetanus toxoid imOf note, a recent study demonstrated the adoptive munization on posttransplant days 88, 113, and 715. transfer of allergen-specific IgEmediated hypersenThus, in this small study, the use of vaccine to trans- sitivity after allogeneic BMT.M This prospective fer adoptive antibody synthesis was successful, and study followed 12 donor/recipient pairs selected for could be augmented by revaccination. donor atopy. The adoptive transfer of allergen-speParenthetically, it is important to stipulate that cific IgE B-cell clones was demonstrated by skin while the measurement of antibody titers facilitates testing in about half of the pairs more than 1 year studies of immune response, the titers may not in& after transplantation. At that time, total IgE levels

vaccines? what techniques can be employed to promo&. optimal effects? Can new vaccines or novel strategies be developed which would actively protect them in the early posttransplant period?

392

April 1995 The American

Journal

of Medicine@

Volume

99

REIMMUNIZATlON

were within the normal range in the 11 patients tested. Of note, those patients with extensive chronic GVHD had significantly fewer positive skin tests, a phenomenon which appeared unrelated to immunosuppressive medications. Thus, donor immunity is adoptively transferred, although the best method for capitalizing on this effect has yet to be determined.

HOW WELL DO BMT PATIENTS RESPOND TO TRADlTlQb#U. VACCINES? Tetanus Vaccine Tetanus toxoid is a protein antigen which is immunogenic even in 6-week-old infants. As described, Wimperis et aP successfully transferred immunity to tetanus toxoid by immunizing both the donor and the recipient prior to transplantation of T-cell-depleted bone marrow. Unforhmately, their study followed the antitetanus titers for only 3 months after BMT, and hence did not evaluate the persistence of the immunity. In a larger study of 48 allogeneic BMT recipients, tetanus toxoid titers were assayed 1 year posttransplant, and the response to tetanus toxoid reimmunization was measured over the subsequent 1 to 2 years.“” Fourteen seronegative and 7 seropositive recipients received one dose of tetanus toxoid 12 months after BMT. One year later, only 7 of the 14 seronegative patients had become seropositive to tetanus toxoid, while all 7 of the seropositive patients maintained their status. Ten of the 14 patients who were initially seronegative were reimmunized with two more doses of tetanus toxoid vaccine between 24 and 36 months post-BMT. All eventually responded, and 900/oremained seropositive 1 year later. The 7 patients seropositive at 1 year after BMT maintained their status for an additional year if given any combination of tetanus toxoid reimmunization. A single booster sufficed. Of note, all 6 patients who were never reimmunized in this study were seronegative to tetanus toxoid by 2 years after BMT, regardless of their initial status. The remaining 21 patier& in this series, 9 of whom were already seropositive, received 3 successive doses of tetanus toxoid vaccine at 12, 13, and 14 months after transplantation. All 2 1 were seropositive 1 year later. Fifteen who were tested 2 years after vaccination all remained seropositive. Interestingly, all seronegative patients, even those with chronic GVHD, responded when given 3 doses of vaccine. Thus, in this study, chronic GVHD did not diminish the responsiveness to tetanus toxoid immunization 1 year after BMT. Since tetanus is not a threat after BMT, it is reasonable to wait until 1 year after transplantation to revaccinate patients in order to be confident of a good response. However, these data reported with

FOLLOWNG BMT/SONIANI AND LARSON

tetanus toxoid vaccine may apply to other protein antigen vaccines as well. If so, giving three successive doses early (perhaps even within 3 to 6 months after BMT) may yield protective imnumity to some common pathogens.

Pneumococcus Vaccine The currently available multivalent pneumococcal vaccine contains pure polysaccharides and requires mature function of the immtme system for maximal response. In 1983, Winston et a.14’jevaluated factors influencing the antibody response to a 14vaIent pneumococcal vaccine in 39 ahogeneic BMT recipients. Twenty-seven patients were vaccinated within 7 months after transplantation and 12 later. Impaired antibody responses were observed in all patients. Male sex, corticosteroid therapy, and early vaccination were associated with a significantly more impaired antibody response. Five patients who were vaccinated early (less than 7 months postoperatively) developed a pneumococcal infection within a mean interval of 119 days after the vaccination. They all survived. All patients vaccinated more than 7 months posttransplant developed protective antibody levels to two or more of the serotypes for which they had previously had inadequate levels. At a minimum, the vaccine itself did not cause morbidity and was protective for some serotypes if given after 7 months. Future studies need to assess the benefit from repeating the vaccine as a booster, or the value of monitoring titers starting at 7 months and continuing for 2 to 3 years. The actual clinical effectiveness of antipneumococcal vaccines may be difficult to validate, since many BMT patients are routinely maintained on prophylactic antibiotics. Giebink et al& studied the response to a 14valent pneumococcal polysaccharide vaccination given after BMT in 10 patients, 7 of whom had received marrow from recently vaccinated donors. Prior to revaccination, the majority of patients had low IgG and IgM antibody titers to pneumococcus. When the recipients were vaccinated 18 to 24 months post-BMT, variable responses were noted with nearly appropriate increases in titers to 2 of the 3 serotypes assayed. Responses to the other 11 serotypes were not measured. Because antibody titers were not followed in all recipients in the early posttransplant period, a protective benefit from vaccinating the donors may have been missed. The investigators found no significant difference in mean antibody titers after vaccination between recipients whose donor had been vaccinated and recipients whose donor had not been vaccinated. The reason for this finding may be twofold. First, the sample size was small. Second and more importantly, the recipients were vaccinated relatively late after BMT. Immunizing the donor prior to transplantation April 1995 The American Journal of Medicine@ Volume 98

393

RElMMUNlZAllON

FOLLOWING

BMT/SOMANI AND LARSON

would ideally enhance the responsiveness of the recipient’s still-immature immune system to early vaccination. Since none of the patients in this study had GVHD by 18 to 24 months after BMT, their immune systems had likely recovered sufficiently to respond independently to the vaccination, diminishing any possible benefit of prior donor immunization. Nevertheless, the responses were still subnormal, and one wonders whether earlier or repeated vaccinations would have an additive effect. Both of the cited polysaccharide pneumococcal vaccine studies showed a suboptimal response within the first 2 years after transplant, even in the absence of GVHD. The poor response may parallel the low serum concentration of the IgG2 isotype documented post-BMT, which has been shown to correlate with a deficient response to bacterial polysaccharide antigens.47 Clearly, up to 7 months, the later the vaccination is given, the better the response is likely to be. Responses have not differed markedly when the vaccine was given between 7 months and 2 years, which is exactly the interval when BMT patients are most susceptible to pneumococcal infection and most transplant centers give prophylactic penicillin or cotrimoxazole. Additional trials are needed to evaluate whether antipneumococcal vaccination of the donor or recipient prior to transplantation can reliably produce adoptive transfer of antibody synthesizing capability or permit successful reimmunization of the recipient soon after BMT. If there is any benefit of administering repeated doses of pneumococcal vaccine during the early posttransplant period to attain a “booster” effect, it still needs to be delineated. Finally, pneumococcal vaccines with stronger immunogenicity are needed. For now, antibiotic prophylaxis should continue, but BMT recipients should receive the pneumococcal vaccine at 7 to 12 months, and perhaps again at 2 years after transplantation. Little information is available regarding the impact of GVHD on the response to the pneumococcal vaccine. Steroid therapy for GVHD hinders the response. Because patients with chronic GVHD have more prolonged IgG2 subclass deficiencies, their response to the pneumococcal vaccine is predictably poorer. Nevertheless, since patients with chronic GVHD are at highest risk for infection, they are candidates for vaccination. Titers should be checked to document an adequate response. If titers are low, a booster should be given.

Polio Vaccine Two studies of the IPV in BMT patients have been reported. Ljungman et al% evaluated immunity to POliovirus and response to IPV in 55 allogeneic tram+ plant patients. About half the seropositive patients demonstrated a significant loss of titers to all 3 po394

April

1995 The American

Journal

of Medicine@

Volume

98

liovirus types by 12 months after transplant. Six seropositive patients were not reimmunized; all became seronegative by 24 to 31 months post-BMT. Nineteen patients were given 1 dose of trivalent IPV 12 months after BMT, while 29 other patients received 3 IPV doses 12, 13, and 14 months after BMT. Those who received the single-dose IPV had poor antibody titers 1 year later, at which time 17 of the 19 received 2 more doses between 24 and 36 months. Their response rates and antibody titers for poliovirus types 2 and 3 increased after 3 total doses of IPV, and were comparable to those in patients who had received 3 doses at months 12 to 14. Overall, patients with GVHD had poorer responses when given either just 1 dose at 12 months or 3 successive doses by month 14. However, patients with chronic GVHD who received 2 additional doses of IPV between 24 and 36 months had final titers significantly higher than after just 1 dose, and their overall response at that point was equal to that of patients without GVHD. This result suggests that chronic GVHD hinders immune recovery and the response to this inactivated viral vaccine during the first year after transplant, but that the problem resolves by the second year. Only 4 of 48 (8%) patients failed to seroconvert despite receiving 3 doses of IPV. Two had GVHD and 2 did not. This study was small and included no controls. Nevertheless, the responses to IPV were clearly subnormal, and all patients required at least three doses to generate an adequate response. Among patients who did not have GVHD, the responses were equal whether the 3 doses were given in rapid succession 12 to 14 months after BMT, or over a total of 24 to 36 months. These data suggest that healthy BMT recipients should receive 3 doses of IPV at 12, 13, and 14 months. Those with chronic GVHD should be reimmunized with 3 doses of IPV at 12,24, and 36 months. Even with this schedule, the responses will not be normal in either group. Engelhard et al* published data from 42 patients who were given 2 consecutive doses of IPV 4 to 6 weeks apart, from 6 to 96 months after BMT, to protect them from a polio outbreak. Antibody titers were measured both before and after vaccination, Antibody titers to all three poliovirus types had declined significantly following BMT, but loss of protective immunity was sign&ant only for types 1 and 3. After the first vaccination, the overall increase in antibody titers to the poliovirus serotypes was statistically significant. The second vaccination led to a further, albeit statistically insignificant, increase in antibody titer levels and an increase in the fraction of patients with immunity. Since the patients were not stratified according to the time from BMT to vaccine administration, this study does not support any optimal time for reimmunization with IPV after RMT.

REIMMUNIZATION FOLLOWING BMT/SOMANI

hong the 25 patients whose sera were available for comparison, the pre-BMT titers correlated with both post-BMT titers and postvaccination titers. Both of the cited polio vaccine studies documented the progressive loss of protective antibody titers to poliovirus following BMT, thus establishing the need for reimmunization. The first concluded that at least three doses of IPV should be given to confer maximal protection, and suggested that the best time for immunization varied depending on the presence of GVHD. In the second study, 8 patients had chronic GVHD, and regression analysis associated GVHD with lower specific antibody titers both after BMT and after revaccination. The oral polio vaccine is a live attenuated virus vaccine that in rare cases has been associated with paralysis in both the vaccine recipients and their contacts. Thus, it should not be given to BMT recipients or any of their household contacts, including young children. Again, since BMT patients in the United States are unlikely to contract polio, early immunization is not urgent. Patients should be reimmunized at some point within the first few years after transplantation. The greater utility of these studies may be as a model for vaccination with other, more pertinent inactivated virus vaccines that are currently available (influenza, hepatitis B), or soon will be (varicella zoster virus, cytomegalovirus).

Mumps-Measles-Rubella

Vaccine

Ijungman et al34 administered MMR vaccine to 20 healthy patients who were free of GVHD and immunosuppressive treatments at least 2 years after BMT with non-T-cell-depleted marrow. He noted no early or late adverse effects. This study established the principle that live virus vaccines are safe in healthy patients 2 years after BMT.

Influenza Vaccine To date, Engelhard et al4g have published the only report of influenza vaccine in BMT patients. Their study included 48 patients, 35 of whom received allogeneic T-cell-depleted marrow. Patients were immunized with 2 doses of influenza vaccine 1 month apart, starting anywhere from 2 to 82 months (median 14.5) after BMT. Protective hemagglutination-inhibition titers were measured. There was a statistically significant association between the time of vaccine administration after BMT and the attainment of protective antibody titers for influenza. No patient vaccinated between 2 and 6 months post-BMT responded with protective titers against any influenza stmins. Three (13%) of 24 who were vaccinated after 7 to 24 months achieved protective titers to all strains; a second vaccination raised this rate slightly, to 21% to 29%. When the vaccine was administered more than

AND LARSON

2 years after BMT, 64% to 71% response rates were achieved for various strains, and a second vaccination produced little added benefit to the overall result. The presence of GVHD affected seroconversion to each of the three influenza strains within the vaccine in a variable fashion. Thus, the overall response to influenza vaccine was poor in BMT patients. Weak responses to this vaccine have also been noted in other immunocompromised groups, such as HIV-positive and renal transplant patients. Engelhard postulates that the frequent changes in the antigens within the influenza vaccine in response to continuous antigenic variation in the wildtype virus may negate any potential benefit from the transfer of donor immun@~.~ This study and the results with MMR vaccine both suggest poor responses overall to these viral vaccines. However, no attempts were made to enhance responses via immunization of the donor or both the donor and recipient prior to transplantation.

Hepatitis B Vaccine The reported experience on the administration of hepatitis B vaccine to BMT patients is slight. This is somewhat surprising in view of the large number of blood transfusions these patients receive. Wimperis et al3g successfully engineered the adoptive transfer of titers to hepatitis B vaccine when both donor and recipient were immunized with at least two (and better yet three) doses of hepatitis B vaccine. Notably, there were only 4 pairs in this group, and they were followed for only 100 days. The data from only a single patient were charted. Much smaller rises in titers were reported in 9 pairs in which only the donor or only the recipient was immunized prior to BMT. One case report in the literature describes the reverse seroconversion of a recipient, previously immune to hepatitis B virus, after receiving marrow from his brother, a chronic hepatitis B virus carrier (HBeAg+, HBsAg+, HBVDNA+).m The patient rapidly developed active hepatitis B virus infection that later progressed to cirrhosis and liver failure.

WHAT ABOUT THE NEW GENERATlON OF VACCINES? The success rate of traditional polysaccharide vaccines in BMT and other immunocompromised patients has been disappointing. The obvious need for more immunogenic vaccines has led to a new generation of polysaccharide-protein coqjugate vaccines51 The impetus for their development rests in part on the failure of the H i@uerwae type b (Hib)-capsular polysaccharide vaccine to protect infants less than 2 years old from meningitis. As a T-cell-independent vaccine, the traditional polysaccharide antigen is not sufficiently immunogenic to the immtiure immune April 1995 The American Journal of Medicine@ Volume 98

395

REWMJWATlObl FOLLOWNG BMT/SOMANI AND LARSON

system. A conjugate vaccine is one in which the polysaccharide antigen is covalently linked with a protein such as tetanus or diphtheria toxoid to form a more immunogenic molecule. Conjugate vaccines evoke a T-cell-dependent response that appears to be stronger in an immature immune system and provides longer lasting immunity.~2 In fact, the Hib-comugate vaccine induces an antibody response even in individuals with primary IgG2 deficiency, leading to speculation that it may successfuRy reimmunize BMT patients who demonstrate this particular deficiency.% The response to the Hib-conjugate vaccine was recently studied in aIlogeneic BMT recipients.p54 Forty patients were enrolled in a randomized, double-blind study that compared the immunogenicity of the standard Hib-capsular polysaccharide vaccine (Hib-CPS) with that of a new tetanus toxoid-conhtgated Hib-CPS vaccine (Hib-CPS-T). Thirty-one patients were immunized between 4 and 18 months following transplantation, and 9 more than 18 months posttransplantation. In the first step, 20 patients received a dose of Hib-CPS, and 20 received a dose of Hib-CPST. AlI patients were then given a dose of Hib-CPST vaccine 1 month later. Serum anti-Hib-CPS antibody levels were measured prior to immunization, 1 month after each injection, and, in 25 cases, 10 to 44 months after the initial immunization. Controls were normal healthy adults who received one injection of either Hib-CPS or Hib-CPST. Prior to any vaccination, 8% of the BMT patients and about 60% of the normal unstimulated controls had protective antibody levels to Hib (>l pg/mL). On day 30,46% of the patients who received Hib-CPS had protective antibody levels, compared to 70% of those who had received Hib-CPS-T. On day 60, both groups demonstrated further increases in total antibody levels. The percentages of patients with protective antibody levels did not differ significantly between the two groups, possibly because of the small size of the study and the relatively long interval between BMT and immunization. The patients who had received two injections of Hib-CPS-T had higher levels than those who received a first injection of Hib-Hib-CPS. Possible long-term benefit due to sustained immunity from Hib-CPS-T in comparison to Hib-CPS cannot be assessed from this study but may be an important advantage to this new conjugate vaccine. A number of patients’ IgG subclasses were fractionated at day 30. IgG2 antibody titers predominated in the Hib-CPS group, and IgGl antibodies in the Hib-CPST group. At day 60, both groups showed a predominance of IgGl-type antibodies. Thus, by stimulating a predominantly IgGl response, these conjugate vaccines may be more immunogenic in BMT patients who have prolonged deficiencies in the IgG2 and IgG4 subclasses. 396

April

1995

The American

Journal

of Medicinea

Volume

98

There was no correlation between antibody response to vaccination and the presence of GVHD, ongoing immunosuppressive therapy, or the time of vaccination up to 18 months after BMT. Response correlated positively with the time of vaccination after 18 months. Because the overall response rate to two Hib-CPS-T vaccinations was 85%, the authors advocate giving a series of three injections of this conjugate vaccine at 6-month intervals. Given the lack of adverse side effects, they recommend starting the series from the fifth month after transplantation in the hope of gaining some protective benefit during the early immunocompromised period. Immunization with a polyvalent P aeruginosa Opolysaccharide-toxin A conjugate vaccine was explored in 14 patients who underwent T-cell-depleted alIogeneic BMT.55 Six donor/recipient pairs each received single immunizations 7 to 10 days prior to BMT. In 4 other pairs, only the donors were immunized, and in another 4 pairs, only the recipients. The vaccine had no reported adverse effects. In the groups where the donor alone or the recipient alone was immunized, there was little antibody response. When both parties were immunized prior to transplantation, the adoptive transfer of an IgG antibody to P amginosa lipopolysaccharide was successfully achieved 2 to 4 weeks after transplantation and remained elevated for up to 3 months. The IgG subclass distribution was delineated in 2 of these recipients and found to consist primarily of IgG2 and IgGl isotypes. Larger trials must be undertaken to confii the findings of this small but encouraging study.

CONCLUSIONS The literature supports the use of vaccinations in allogeneic BMT patients, and standard vaccines can be administered effectively.““i More comprehensive data need to be gathered concerning the optimal methods and timing of immunization. Future research should be directed toward creating vaccines with greater immunogenicity and identifying the conditions that reliably transfer immunity from donor to host. The goal is earlier, more clinically beneficial immunization. Furthermore, vaccines that protect against P aeruginosa, varicella zoster virus, and cytomegalovirus, the most common and morbid infections after BMT, should be vigorously pumued. New molecules that could have value for BMT patients are awaiting investigation. For example, recombinant vaccinia virus vaccines that express various epitopes of the streptococcal M protein have been shown to be immunogenic and protective in animal studiesZ7 Live virus vaccines are known to elicit strong and sustained responses, but their use in immunocomprondsed patients is not recommended at present. Efforts t,o further att,enuate the live vaccinia

vector are being pursued and may allow its eventual use in BMT patients. In the more immediate future, the conjugate vaccines promise to be highly immunogenic, and should be diligently investigated in this special population. Recommendations for a reimmunization schedule remain tentative because the literature is sparse, the studies often limited, and the conclusions correspondingly fragile. Based on immune recovery data, most healthy recipients should be capable of responding normally by 2 years after transplantation. However, the optimal time to immunize depends only partly on the level of antibody response, and partly on the patient’s underlying risk of infection. Waiting 2 years will ensure good antibody production but miss the opportunity to protect patients during the critical period immediately post-BMT, when their immune systems have not yet redeveloped. Admimstmtion of repeated vaccine doses soon after transplantation is more likely to increase protection and decrease morbidity. Other than cost, there appears to be no disadvantage to early and repeated vaccinations. S pwumoniae and H inJhn;zm infections most often occur within the first 1 to 2 years, and hence are logical candidates for early vaccination. Similarly, if vaccines for cytomegalovirus or P awuginosa are perfected, one would seek to administer them as soon as possible. In contrast, since transplant recipients are not known to have significant morbidity from diphtheria, tetanus, or polio, vaccines against these pathogens could be administered at 1 year. No harm has been reported from immunizations in BMT recipients, although the subject has not been the focus of any study to date. The assessment of benefits and potential harm from early vaccinations after BMT will require prospective clinical trials. Recognizing the strengths and the shortcomings of the literature, the following clinical guidelines are currently practiced at our institution. Revisions are expected as more data accumulate. 1. Vaccinate patients regardless of their GVHD status (with the exception of live virus vaccines). 2. Administer pneumococcal vaccine at 7 months and again at 24 months post-BMT. 3. Give 3 doses of Hib-conjugate vaccine 6 months apart, starting at 7 months. 4. Reimmunize with diphtheria and tetanus toxoids after 12 months. 5. Give three doses of enhanced inactivated poliovirus vaccine (eIPV) at l-month intervals starting at 12 months post-BMT. In patients with chronic GVHD, vaccinate instead at 12, 24, and 36 months. Three doses of hepatitis B vaccine can be given on the same schedule. 6. Where possible, check titers after 3 months and revaccinate if necessary.

7. Administer the current influenza vaccine every autumn. 8. When possible, use conjugate vaccines. 9. Do not use live virus vaccines before 2 years. 10. At 2 years, MMR vaccine should be given to healthy BMT patients free of GVHD. 11. Do not give the oral poliovirus vaccine to BMT patients or members of their households.

ACKMOWLEDGIWENT The authors thank Dr. John Flaherty for helpful discussions and Susan Jarman for preparation of the manuscript.

REFERENCES 1. Witherspoon RP. Suppression and recovery of immunologic function after bone marrow transplantation. J Nati Cancer Inst. 1986;76:1321-1324. 2. Witherspoon RP, Deeg HJ, Lum LG, et al. Immunologic recovery in human marrow graft recipients gwen cyclosporine or methotrexate for the prevention of graft-versushost disease. Jransplantahon. 1984;37:456-461, 3. Lum LG. The kinetics of immune reconstitution after human marrow transplantation. Blood. 1987;69:369-380. 4. Witherspoon RP, Storb R, Ochs HD. et al. Recovery of antibody production tn human allogeneic marrow graft recipients: influence of time posttransplantation, the presence or absence of chronic graft-versus-host disease, and antithymocyte globulin treatment. Blood. 1981;58:360-368. 5. Kiesel S, Pezzutto A, Moldenhauer G, et al. B-Cell proliferative and differentiative responses after autologous peripheral blood stem cell or bone marrow transplantation Blood. 1988;72:672-678. 6. Shiobara S, Harada M, Mori T, et al. Difference in posttransplant recovery of Immune reactivity between ailogenerc and autologous bone marrow transplantation. Transplant hoc. 1982;14:429-433. 7. Reittie JE, Poulter LW, Prentice HG, et al. Differential recovery of phenotypically and functionally distinct circulahng anhgenpresenting cells after allogeneic marrow transplantation. Transplantation. 1988;45:1084-1091. 8. Nrederwieser D, Gasti G, Rumpold H, et al. Raprd reappearance of large granular lymphocytes with concomitant reconstitution of natural killer activrty after human bone marrow transplantation. Br J Haematol. 1987;65:301-305. 9. Shiobara S, Witherspoon RP, Lum LG, Storb R. Immunoglobulin syntheses after HLArdentical marrow grafting. V. The role of peripheral blood monocytes in the regulahon of in vitro immunoglobulin secrehon stimulated by pokeweed mrtogen. J Immunol. 1984;132:2850-2856. 10. Livnat S, Seigneuret M, Storb R, Prenhce RL. Analysis of cytotoxrc effector cell function in patients with leukemia or aplashc anemia before and after marrow transplantation. J Immunol. 1980;124:481-490. 11. Zander AR, Reuben JM, Johnston D, et al. Immune recovery followrng allogenerc bone marrow transplantation. Transplantation. 1985;40:177-183. 12. Hokland M, Jacobsen N, Ellegaard J, Hokland P. Natural killer funchon following allogeneic bone marrow transplantatron. Transplantation. 1988;45:1080-1084. 13. Fass L, Ochs HD, Thomas ED, et al. Studies of immunologic reactivrty following syngeneic or allogeneic marrow grafts in man. Transplantation. 1973;16:630-640. 14. Matsue K, Lum LG, Witherspoon RP, Storb R. Proliferatrve and drfferentiatrve responses of B cells from human marrow graft recipients to Tcell derrved factors. Blood. 1987;69:30&315. 15. Elfenbein GJ, Anderson PN, Humphrey RL, et al. Immune system reconstitution following allogeneic bone marrow transplantation rn man: a multiparameter analysis. Transplant Proc. 1976;8:641-646. 16. Witherspoon RP, Lum LG, Storb R, Thomas ED. In vitro regulation of rmmunoglobulin syntheses after human marrow transplantation. II. Deficrent T and noni lymphocyte function within 34 months of allogeneic, syngenerc, and autologous marrow grafting for hematologic malignancy. Blood. 1982;59:844-850. 17. Ringden 0, Witherspoon RP, Storb R. et al. B cell functron in human marrow transplant recrpients assessed by direct and Indirect hemolysrs-In-gel

April 1995 The American Journal of Medicine@ Volume 98

397

RElh’lMlJNIZATiON

FOLLOWING

BMT/SOMANI

AND

LARSON

assays. J Immunol. 1979;123:2729-2734. 18. Korsmeyer SJ, Elfenbein GJ, Goldman CK, et al. B cell, helper T cell, and suppressor T cell abnormalities contribute to disordered immunoglobulin synthesis In patients following bone marrow transplantation. Transplantation. 1982;33:184-189. 19. Lum LG, Seigneuret MC, Storb R, et al. In vitro regulation of immunoglobulin synthesis after marrow transplantation. T-cell and B-cell deficiencies in patients with and without chronic graft-versus-host disease. Blood. 1981;58:431-439. 20. Lum LG, Seigneuret MC, Orcutt-Thordarson N, et al. The regulation of immunoglobulin synthesis after HLA-Identical bone marrow transplantation. VI. Differential rates of maturation of distinct functional groups within lymphoid subpopulations in patients after human marrow grafting. Blood. 1985; 65:1422-1433. 21. Kagan JM, Champion RE, Saxon A. B-cell dysfunction followlng human bone marrow transplantation: functional-phenotypic dissociation during the early posttransplant period. Blood. 1989;74:777-785. 22. Witherspoon RP, Goehle S, Kretschmer M, Storb R. Regulation of lmmunoglobulin production after human marrow grafting. The role of helper and suppressor T cells in acute graft-versus-host disease. Transplantation. 1986;41:328-335. 23. Linch DC, Knott LJ, Thomas RM, et al. T cell regeneration after allogeneic and autologous bone marrow transplantation. Br J Haematol. 1983;53: 451-458. 24. Forman SJ, Niicker P, Gallagher M, et al. Pattern of T cell reconstitution following allogeneic bone marrow transplantation for acute hematological malignancy. Transplantation. 1982;34:96-98. 25. de Bruin HG, Astaldi A, Leupers T. et al. T lymphocyte characteristics in bone marrow-transplanted patients. J Immunol. 1981;127:244-251. 26. Fox R, McMillan R, Spruce W, et al. Analysis of T lymphocytes after bone marrow transplantation using monoclonal antibodies. Blood. 1982; 60:57&582. 27. Saxon A, McIntyre RE, Stevens RH, Gale RP. Lymphocyte dysfunction in chronic graft-versus-host disease. Blood. 1981;58:746-751. 28. Lum LG, Orcuti-Thordarson N, Seigneuret MC, Storb R. The regulation of Ig synthesis after marrow transplantation. IV. T4 and T8 subset function in patients with chronic graft-versus-host dtsease. J Immunol. 1982:129: 113-119. 29. Wiiherspoon RP, Matthews D, Storb R, et al. Recovery of in VIVO cellular Immunity after human marrow grafting. Influence of time postgrafting and acute graft-versushost disease. Transplantation. 1984;37:145-150, 30. Velardl A, Cuccialoni S, Terenzi A, et al. Acquisitton of Ig isotope dlversity after bone marrow transplantation in adults. A recapitulation of normal B cell ontogeny. J Immunol. 1988;141:815-820. 31. Aucouturier P, Barra A, lntrator L, et al. Long lasting IgG subclass and antibacterial polysaccharide antlbody deficiency after allogenelc bone marrow transplantation. Blood. 1987;70:779-785. 32. Storek J, Ferrara S, Ku N, et al. B cell reconstitution after human bone marrow transplantation: recapitulation of ontogeny? Bone Marrow Transplant. 1993;12:387-398. 33. Wlngard JR. Advances in the management of infectious complications after bone marrow transplantation. Bone Marrow Transplant. 1990;6:371-383. 34. Ljungman P, Fridell E, LGnnqvist B, et al. Efficacy and safety of vaccination of marrow transplant recipients with a I’ve attenuated measles, mumps, and rubella vaccine. J Infect Dis. 1989;159:610-615. 35. Ljungman P, Wlklund-Hammarsten M, Dura] V, et al. Response to tetanus toxoid immunization after allogeneic bone marrow transplantation. J Infect Dls. 1990;162:496-500, 36. Lum LG, Munn NA, Schanfleld MS, Storb R. The detection of specific antibody formation to recall antigens after human bone marrow transplantation. Blood. 1986;67:582-587.

398

April 1995

The American

Journal

of Medicinea

Volume

98

37. Lum LG, Noges JE, Beatty P, et al. Transfer of specific immunity In marrow recipients given Hi-A-mismatched, T cell-depleted or HLA identical marrow grafts. Bone Marrow Transplant. 1988;3:399-406. 38. Ljungman P, Duraj V, Magnius L. Response to immunization against polio after allogeneic marrow transplantation. Bone Marrow Transplant. 1991;7:89-93. 39. Wlmperis JZ, Brenner MK, Prentice HG, et al. Transfer of a functlonlng humoral immune system in transplantation of T-lymphocyte-depleted bone marrow. Lancet. 1986;1:339-343. 40. Wahren 8, Gahrton G, Linde A, et al. Transfer and persistence of viral antibody-producing cells in bone marrow transplantation. J Infect Dis. 1984;150:358-365. 41. Saxon A, Mitsuyasu R, Stevens R, et al. Designed transfer of specific immune responses with bone marrow transplantation. J Clin Invest. 1986;78:959-967. 42. Wimperis JZ, Gottlieb D, Duncombe AS, et al. Requirements for the adoptwe transfer of antibody responses to a priming antigen In man. J Immuflol. 1990;144:541-547. 43. Kodo H, Gale RP, Saxon A. Antibody synthesis by bone marrow cells in vitro following primary and booster tetanus toxotd immunlzatlon in humans. J Clm Invest. 1984;73:1377-1384. 44. Agosti JM, Sprenger JD, Lum LG, et al. Transfer of allergen-specific IgEmediated hypersensitivity with allogeneic bone marrow transplantation. NEJM. 1988;319:1623-1628. 45. Winston DJ, Ho WG, Schiffman G, et al. Pneumococcal vaccination of recipients of bone marrow transplants. Arch Intern Med. 1983;143: 17351737. 46. Gieblnk GS. Warkentin PI, Ramsay NKC, Kersey JH. Titers of antibody to pneumococci in allogeneic bone marrow transplant recipients before and after vaccination with pneumococcal vaccine. J Infect DIS. 1986;154:590-596. 47. Slber GR, Schur PH, Aisenberg AC, et al. Correlation between serum lgG2 concentrations and the antibody response to bacterial polysaccharide antigens. NEJM. 1980;303:178-182. 48. Engelhard 0, Handsher R, Naparstek I, et al. Immune response to polio vaccination in bone marrow transplant recipients. Bone Marrow Transplant. 1991;8:295-300. 49. Engelhard D, Nagler A, Hardan I, et al. Antibody response to a twodose regimen of influenza vaccine in allogeneic T-cell depleted and autologous BMT recipients. Bone Marrow Transplant 1993;11:1-5. 50. Fan FS, Tzeng CH, Yeh HM, Chen PM. Reverse seroconversion of hepatitis B virus infectious status after allogenelc bone marrow transplantation from a carrier donor. Bone Marrow Transplant. 1992;10:189-191. 51. Robbins JB, Schneerson R. Polysaccharide-protein conjugates: a new generation of vaccines. J Infect Dis. 1990;161:821-832. 52. Chu C, Schneerson R, Robbins JB, Rastogi SC. Further studies on the lmmunogen&y of Haemophilus influenza type b and pneumococcal type 6A polysaccharide protein conjugates. Infect Immunol. 1983;40:245-256. 53. Ambrosino DM. Impaired polysaccharide responses in immunodeficlent patients: relevance to bone marrow transplant patients. Bone Marrow Transplant 1991;7(suppl 3):48-51. 54. Barra A, Cordonnier C, Preziosl MP, et al. lmmunogenlclty of Hemophrlus influenza Type b conjugate vaccine in allogeneic bone marrow recipients. J Infect DE. 1992;166:1021-1208. 55. Gottlieb DJ, Cryz SJ, Furer E, et al. lmmunlty against Pseudomonas aeruginosa adoptlvely transferred to bone marrow transplant reclplents. Blood. 1990;76:2470-2475, 56. Ambroslno DM, Molrlne DC. CrItical appraisal of immunization strategies for preventron of InfectIon in the compromised host. Hematol Oncol Clan North Am. 1993;7:1027-1049, 57. Hruby DE. Vaccinia virus: a novel approach for molecular engineering of peptide vaccines. Semin Hematol. 1993;3O(suppl4):35-43.