Impairment of the thymus-dependent humoral immune response by syngeneic or allogeneic pregnancy

Journal of Reproductive Immunology, 4 (1982) 337-348

337

Elsevier Biomedical Press

Impairment of the thymus-dependent humoral immune response by syngeneic or allogeneic pregnancy M . G . Baines * and H.F. Pross Department of Microbiology and Immunology, McGill University, 3775 University St., Montreal, Quebec, Canada H3A 2B4, and Departments of Radiation Oncology and Microbiology and Immunology, Queen's University, Kingston, Ontario, Canada K7L 3N6

(Received 3 November 1981; accepted 8 April 1982)

Pregnancy results in a diminished capacity of the gravid mouse to respond to thymus-dependent antigens. The IgM response to thymus dependent and independent antigens is relatively unaltered whereas the IgG response to thymus-dependent antigens is markedly suppressed in both syngeneic and allogeneic pregnancy. The impairment of the humoral immune response is not due to loss of antibodyforming precursor lymphocytes (AFPL) from the spleen since their number remains unaltered. During pregnancy the spleen becomes enlarged and the precursor frequency is diluted somewhat but the AFPLs retain their normal activity in vitro. This implies an alteration in their amplification by accessory cells (T cells or macrophages) is responsible for the impaired IgG responses.

Introduction T h e ability of a m a m m a l i a n fetus to avoid i m m u n e rejection d u r i n g g e s t a t i o n r e m a i n s an intriguing p a r a d o x in current i m m u n o l o g y . D u r i n g p r e g n a n c y the m o t h e r r e m a i n s i m m u n o c o m p e t e n t with respect to foreign p a t h o g e n s a n d yet m u s t s o m e h o w o v e r l o o k the a l l o a n t i g e n s expressed b y the fetus a n d p l a c e n t a . M a n y theories have b e e n a d v a n c e d to explain the u n u s u a l a c c e p t a n c e of the fetal allograft b y the m a t e r n a l i m m u n o c o m p e t e n t system (Beer a n d Billingham, 1971; A n d e r s o n , 1977) which generally fall into two categories: (a) afferent failure of the i m m u n e r e s p o n s e or (b) central failure of the i m m u n e response. Efferent failure of the i m m u n e r e s p o n s e m a y also be significant since it has been shown that alloimm u n e a n t i b o d i e s are a b s o r b e d b y the p l a c e n t a a n d e l i m i n a t e d w i t h o u t a n y h a r m f u l effect to the fetus ( W e g m a n n et al., 1979). C e r t a i n l y the placenta, a n d the t r o p h o b l a s t layer in p a r t i c u l a r , has a m a j o r role to p l a y in p r o t e c t i n g the fetus f r o m i m m u n e cells a n d a n t i b o d i e s which could cause the rejection of a n o r m a l allograft. Since this s t u d y has e x a m i n e d the effect of p r e g n a n c y on the p r o d u c t i o n of h u m o r a l a n t i b o d i e s to test antigens, further c o n s i d e r a t i o n of the p r o t e c t i v e role of the * To whom reprint requests should be addressed. 0165-0378/82/0000-0000/$02.75

© 1982 Elsevier Biomedical Press

338 placenta will not be considered in this paper. Afferent failure may be explained by either modulation and deletion of fetal transplantation antigens (Uphoff, 1970) or, masking of the fetal membrane antigens with an immunologically inert sialomucin (Currie and Bagshawe, 1967; Kirby, 1968). On the other hand, central failure may be due to either immunosuppression by pregnancy-associated steroids (Carter, 1976; Fabris, 1973) or immunoregulation by suppressor cells (Clark and McDermott, 1978; Rees et al., 1975; Hellstrom and Hellstrom, 1975) a n d / o r blocking factors (Hellstrom et al., 1969; Heron, 1972; Kasakura, 1971). There have been several publications indicating impairment of ceU-mediated immunity during pregnancy (Carter, 1976; Fabris, 1973; Smith et al., 1972) but very few showing whether the humoral immune response is also affected (Fabris, 1973; Baines et al., 1977a; Suzuki and Tomasi, 1979). In addition, there appears to be some confusion with respect to the nature of the suppression of antibody responses by pregnancy. Using a variety of thymus-dependent and independent antigens we have sought to investigate the effect of pregnancy on the humoral immunocompetence of the mouse. The results indicate that antibody precursor B cells and IgM responses remain unaltered by pregnancy whereas IgG responses to thymus-dependent antigens are often depressed.

Methodology Animals

Groups of seven female C57B1/6J mice were mated by either D B A / 2 J or C57B1/6J males and the date of vaginal plugs was noted. 15-18 days later primigravid females and virgin controls were immunized either intravenously or intraperitoneally with challenge antigens. Other strains used in these studies are indicated where appropriate. A n tigens

The thymus-independent antigens E. coli 0.55.B5 lipopolysaccharide (LPS) (Difco), Ficol1400 (Pharmacia) and thymus-dependent antigens sheep red blood cells (SRBC), ox red blood cells (OX) and bovine gamma-globulin (BGG) were used in these studies. Mice were immunized intravenously or intraperitoneally with 0.2 ml of the antigens as indicated in the text. The soluble carriers (LPS, Ficoll and BGG) were coupled with dinitrophenyl (DNP) or trinitrophenyl (TNP) haptens prior to use as immunogens to facilitate subsequent assay. Using the dichlorotriazinyllysine derivative of TNP-lysine (TNPLy), TNP was coupled to either the immunogen or the indicator erythrocytes (Blakeslee and Baines, 1978). The molar ratios of the immunogens were calculated using the published extinction coefficients for T N P (E350 = 15400, Rittenberg and Amkraut, 1966) and the following molecular weights for the carriers: LPS = 190000; Ficoll= 400000 and B G G = 160000. The molar ratios of the antigens used are indicated in the text. To enhance the immunogenicity of the T N P - B G G it was

339 necessary to prepare an alum precipitate prior to intraperitoneal immunization (Campbell et al., 1970).

Assays of the humoral response At predetermined days after immunization the mice were anaesthetized and exsanguinated. The serum was separated and titrated against the relevant indicator erythrocytes for total and 2-mercaptoethanol (2-ME) resistant antibodies by a standard micro-hemagglutination titration technique. After recording t h e total hemagglutinin titer 25 /~1 of 0.1 M 2-ME was added to each well and the pellets resuspended by vortexing to allow the estimation of the 2-ME-resistant titers. The spleens were also excised from these mice and were assayed for plaque-forming cells against unsensitized sheep erythrocytes or sheep erythrocytes sensitized with the appropriate hapten in the conventional Jerne plaque assay (Baines et al., 1977; Blakeslee and Baines, 1978). TNP-sulfonic acid was used to sensitize the indicator cells (Rittenberg and Pratt, 1969). The respective antigens and indicators are shown in the text and tables. The polyspecific rabbit anti-mouse immunoglobulin developing serum used to visualize IgG plaques had an IgM inhibition constant of 0.8 and an IgG development constant of 1.3 and the results were expressed as mean plaque-forming cells per spleen (Baines et al., 1977). Student's 't '-test was performed on the logarithmically transformed results and geometric means were computed for the groups of mice.

Limiting dilution analysis The presence of immunocompetent cells was detected in an in vitro limiting dilution assay to quantify the number of antigen-reactive units (ARU) in the spleens of normal and pregnant mice. The method of Lefkovits (1979) was used as adapted to a semi-micro culture system. Briefly, spleen cells, starting with 1 × 106 per well in 100 /~1 replicates of 12, were serially diluted in round bottom Linbro (Flow Labs, Rockville, Md.) microtiter trays and 100/~1 of 0.1% sheep erythrocytes (SRBC) were added to each well. The medium used was RPMI-1640 (Flow Labs) containing 20% fetal calf serum, 2 mM glutamine, 25 mM Hepes and 4 × 10 5 M 2-mercaptoethanol. After 6 days incubation at 37°C in a 5% CO 2 atmosphere the supernatants were assayed in the presence of guinea pig complement for anti-sheep erythrocyte hemolytic antibody and the cell pellets were assayed for plaque forming cells on a thin agarose layer containing SRBC. Computation of ARU frequency employed the Poisson distribution according to Lefkovits (1979). Results

The acceptance of the fetal allograft would appear to indicate that specific or non-specific immunosuppression accompanies pregnancy. The challenge of pregnant mice with a thymus-dependent antigen often demonstrated an impairment of the IgG humoral immune response. This was regularly seen in mice which were either syngeneically or allogeneically pregnant between the 15th day of pregnancy and term.

340 TABLE 1 Responses to thymus-dependent antigens during syngeneic pregnancy (geometric mean plaque-forming cells/spleen) ~ Type of mice and antigen

Day tested

C57B1/6J (2% SRBC I / V )

5

Type of response

IgM lgG

C57B1/6J (2% SRBC I / V )

6

IgM IgG

BCF1 (2% SRBC I / V )

6

IgM IgG

Status of mice tested Virgin

Day 18 of pregnancy

12589 b (4,10-~ 0.09) 2291 (3.36--+0.09) 130275 (5.11~+0.06) 80797 (4.91+0.06) 23 475 (4.37+0.06) 31798 (4.50+0.18)

11 748 (4.07-+0.10) 794 (2.90-+0.05) 130714 (5.12+0:04) 25 571 (4.41--+0.12) 5 248 (3.72+0.28) 3236 (3.51 +0.27)

Significance by Student's 't'-test

n.s. <0.01 n.s. <0.05 n.s. <0.05

Each group consisted of 6-12 mice mated once at 8-12 weeks of age by a syngeneic male. Pregnancies were timed from the day the vaginal plug was seen (day 0). SRBC. sheep red blood cells; n.s., not significant. b Geometric mean (logarithmic mean - 1 S.E.).

Using particulate antigens (sheep erythrocytes) syngeneically pregnant mice were challenged and their IgG response was found to be significantly lower than that of age-matched virgin mice (Table 1). When allogeneically pregnant mice were similarly challenged the majority demonstrated a significant impairment of their IgG responses (Table 2). The effect of allogeneic pregnancy upon the response to soluble antigen was tested by challenging gravid mice at the 15th to 18th day of pregnancy with TNP-BGG with alum. Both IgG and IgM responses were seen to be lower than normal though the IgG response was most significantly impaired to both antigen preparations tested (Table 3). These observations showed that the immune response to both soluble and particulate antigens was susceptible to the same defect indicating that the form of the antigen and presumably antigen processing were not critical factors. However, it was noted that the IgM response to the soluble antigen TNP-BGG was also impaired during pregnancy, indicating perhaps pregnancy-associated inhibitory effects on the adjuvant induced humoral immune response to TNP-BGG (Table 3). The kinetics of the immune response was examined to ensure that the peak of the response had not been displaced by pregnancy. Immunization during either syngeneic or allogeneic pregnancy showed that the time course was identical to control responses though generally the magnitude of the response was lower than thflt of the virgin controls (Fig. 1). These observations indicated a non-specific impairment of

341

TABLE 2 Responses to thymus-dependent antigens during allogeneic pregnancy (geometric mean plaque-forming cells/spleen) a Mating

C57B1/6J X DBA/2J

Day assayed

Type of plaque

5

IgM IgG

CBA/J X DBA/2J

6

IgM IgG

CBA/J X DBA/2J

7

IgM IgG

C57B 1/6J X DBA/2J

8

IgM IgG

Status of mice Virgin

Day 18 of pregnancy

12589 b (4.10--+0.09) 2298 (3.36-+0.23) 25 853 (4.41 -+0.06) 203 862 (5.31 -+0.08) 8 110 (3.91 + 0.08) 65 642 (4.82 + 0.07) 5 916 (3.77-+0.15) 6 896 (3.83-+0.09)

11 749 (4.07±0.10) 2512 (3.40+0.14) 22 142 (4.35 +0.12) 31285 (4.50±0.22) 9 302 (3.97 --+0.09) 22133 (4.35 ± 0.03) 4 872 (3.69--+0.14) 2 320 (3.36+0.13)

Significance of Student's 't'-test

n.s. n.s. n.s. <0.01 n.s. <0.01 n.s. <0.05

a Each group consisted of 6 - 1 2 female mice mated once at 10-14 weeks of age. Pregnancies were timed from the day the vaginal plug was seen (day 0) and were challenged with 0.2 ml of 2% sheep erythrocytes on day 18-+ 1 of gestation. b Geometric mean (logarithm of mean ± 1 S.E.).

TABLE 3 The effect of pregnancy on the maternal immune response for a thymus-dependent antigen (TNP-Ly)BGG a Immunogen (0.5 m g 1 / P with alum)

(TNP-Ly)9BGG (TNP-Ly)lTBGG

Geometric mean T N P plaque-forming cells/spleen Normal virgin C 5 7 B I / 6 J

Pregnant C 5 7 B 1 / 6 J +

IgM

IgG

IgM

IgG

2470 (3.39--+0.08) b 3369 (3.52 --+0.10)

1407 (3.15 ~0.16) 9509 (3.98 --+0.09)

837 (2.92_+0.23) 1751 * (3.24 ± 0.08)

130 *** (2.12--+0.15) 1 557 ** (3.19 + 0.20)

a Each group contains 9-11 female mice. The virgins were age matched with the gravid females which were in their 15th to 18th day of pregnancy by a D B A / 2 J male when immunized and they were assayed against T N P - S R B C on day 7. (TNP-Ly)-BGG; bovine g a m m a globulin conjugated with the trinitrophenyl-lysine hapten (Blakeslee and Baines, 1978). b Logarithm of mean z 1 S.E. * P <0.05; ** P < 0 . 0 1 ; *** P < 0 . 0 0 1 .

342

÷

///! /

E

//

i

2

',\'4", 4

6

8

10

12

14

Tmme in days Fig. I. Time course response of C B A / J mice challenged intravenously with 0.2 ml of 2% sheep erythrocytes. The IgM (open symbols, dashed lines) and IgG (closed symbols, continuous lines) plaqueforming cell responses were assayed in virgin (circles), syngeneically pregnant (triangles) and allogeneically (DBA/2J) pregnant (squares) mice at day 17-+1 day of gestation. The IgG responses are significantly lower on days 6, 10, 12, 14 for syngeneic pregnancies and on days 6 and 8 for allogeneic pregnancies.

TABLE 4 Responses to the thymus-independent antigen TNPs-LPS during pregnancy Expt. No.

Allogeneic matings

Antigen

Day 4 IgM plaque-forming cells/spleen

Serum titer ~

1

C57B1/10J× DBA/2J

1 fig TNPs-LPS

129 144 b (5.11 -~0.14) 140961 (5.15 ± 0.04)

4.8/0

4940 (3.69--+0.15) 3 300 (3.52-+0.21)

3.0/0

C57B1/10J virgins 2

C57BI/10J×DBA/2J C57B 1/ 10J virgins

0.1 fig TNPs-LPS

5.4/0

4.7/0

a Serum titers are expressed as the mean negative log 2 of the last serum dilution to show positive agglutination of TNP-modified sheep erythrocytes. Total antibody titer is expressed over the 2mercaptoethanol-resistant antibody titer. TNPs-LPS, E. coli lipopolysaccharide coupled with 8 TNP groups per 100000 daltons using TNP-sulfonic acid (Rittenberg and Pratt, 1969). b Geometric mean (logarithm of mean -+ 1 S.E.).

343 TABLE 5 Responses to the thymus-independent antigen (DNP-Ly)22-Ficoll 400 during pregnancy a Dose of antigen

Geometric mean day 6 maternal immune response Virgin C57BI/6J

0.1 /~g I / V 5.0/x g I / V

Day 15 ( + 1) pregnant C57B1/6J

IgM pfc/spleen

Titer b

IgM pfc/spleen

Titer

17339 (4.24-+0.06) c 139 895 (5.15±0.06)

4.6/0

27647 (4.44±0.12) 143 792 (5.16+0.08)

5.1/0

6.7/0

6.7/0

a Groups of 8-10 mice were used as virgin or pregnant subjects which were mated by DBA/2 males and timed from the day the vaginal plug was seen. (DNP-Ly)22-Ficoll 400, Ficoll (molecular weight 400000) conjugated with dinitrophenyMysine (molar ratio 22) by the cyanuric chloride method (Blakeslee and Baines, 1978). b Total and 2-mercaptoethanol resistant titers were assayed using TNP-SRBC as indicated in the methods and subscript to Table 4. c Logarithm of the mean and S.E.

the thymus-dependent IgG response which accompanied the physiological and hormonal alterations of pregnancy. It was noted that the IgM responses (Fig. 1, Tables 1-3) were relatively unaffected by pregnancy. This was confirmed by an examination of the response of pregnant mice to thymus-independent antigens. Table 4 shows that the response to haptenated bacterial lipopolysaccharide was unimpaired by allogeneic pregnancy, while Table 5 confirms this using haptenated Ficoll as a thymus-independent antigen. Table 6 shows, not only that suppression was not seen to thymus-independent TABLE 6 Time course response to a thymus-independent antigen a Group challenged with antigen

1. Pregnant mice 2. Virgin control mice

Geometric mean response of C57B1/6 mice to (DNP-Ly)22-Ficoll. IgM plaques/spleen after challenge on day 0 Day 4

Day 5

Day 6

Day 7

6420 (3.81 +0.07) b 5 032 (3.70--+0.09)

61 270 (4.79±0.07) 41 391 (4.62±0.11)

25628 * (4.41 ± 0.10) 11 936 (4.08+0.10)

16 104 (4.21 ±0.19) 5 724 (3.76±0.10)

Groups of 4-8 C57B1/6J mice were challenged intravenously with 0.1 /~g (DNP-Ly)22-Ficoll and assayed for spleen plaque forming cells against TNP-SRBC. Pregnant mice had been mated by C57B 1/6J males 16-18 days previously. b Logarithm of mean = 1 S.E. * P <0.05 versus virgin controls. a

344 TABLE 7 Dilution of precursor B-lymphocytes by pregnancy, In vitro limiting dilution analysis of spleen lymphocytes by direct plaque assay a Spleen cell donors ( n )

Virgin controls (18) 14-+ 1 day of pregnancy (18)

A R U i n m u r i n e spleens to sheep erythrocytes Spleen cells × 106

ARU/t06

ARU/spleen

105 -+ 6 151 ± 9 ***

2.86 -+ 0.25 2.08±0.25 *

285 -+ 26 300±38

Syngeneic and allogeneic mating were pooled as their results were identical. C57B1/10J female mice were mated with syngeneic C57BI/10J males or allogeneic D B A / 2 J males. A R U , antigen-reactive units. * P <0.05 versus virgin controls. *** P <0.001 versus virgin controls.

antigens in syngeneic pregnancy, but also that the time course of the response was not altered. Therefore, the IgM response was unaffected by the physiological and endocrinological changes accompanying pregnancy, indicating that maturation of immunocompetent cells was unaffected. These observations could indicate a depletion of antigen-reactive units from the Spleen during pregnancy. Certainly the spleen first greatly enlarged during early pregnancy and then returned to normal size coincident with the demonstration of non-specific suppression (Table 7). The use of in vitro limiting dilution analysis showed that alterations in ARU frequency were demonstrable during pregnancy; however, the total ARU per spleen was not significantly altered, indicating no depletion of, at least, precursors to IgM. This lack of depletion was confirmed at days 7, 14 and 18 of syngeneic or allogeneic pregnancy (Table 8). This observation was also seen when additional irradiated helper cells (irradiated allogeneic spleen

TABLE 8 Lack of depletion of splenic B-lymphocytes during pregnancy. In vitro limiting dilution analysis of spleen lymphocytes by direct plaque assay a Spleen cell donors

Virgin controls 7-+ 1 days of pregnancy 14± 1 days of pregnancy 1 8 ± 1 days of pregnancy

A R U to sheep erythrocytes in murine spleens Syngeneic mating

Allogeneic mating

320±32 259~+55 295+58 281--73

286-+41 171-+64 252±39 202-+32

(18) (7) (11) (5)

(15) (3) (10) (8)

a For the purpose of statistical comparison the control groups were pooled but no significant difference was seen between the virgins and either of the pregnant groups. Syngeneic matings utilized C57BI/6J males and females. Allogeneic matings utilized C57B1/6J females and D B A / 2 J males. The numbers in parentheses indicate the numbers Qf mice assayed.

345 cells) were added to the cultures to ensure excess help (data not shown). Therefore, spleens of pregnant mice did not lack precursor B-lymphocytes and yet the response in vivo was impaired. These observations do not mean that the pregnant female was immunologically deficient. The antibody-forming cell response is only partially though significantly impaired (Fig. 1). The IgM and IgG humoral antibody titers to both thymus-dependent and -independent antigens were not significantly depressed. This observation could be due to a more dispersed production of serum antibodies coupled with the fact that serum titration is a much less sensitive assay of the immune response and therefore is less likely to identify small changes in responsiveness. Finally, it was noted that the suppressive effect was less marked in allogeneic pregnancy as opposed to syngeneic pregnancy (Fig. 1, Table 2).

Discussion

Several points have been demonstrated by this study of humoral immunocompetence in pregnant mice. First, pregnancy impairs the ability of the maternal immune system to produce an IgG response to thymus-dependent antigens. Second, the IgM response to thymus dependent or independent antigens is relatively unaffected. Third, this non-specific effect is perhaps more easily demonstrable during syngeneic pregnancy than during allogeneic pregnancy. Finally, the pregnancy does not alter the numbers of antibody precursor cells in the spleen. It has often been said that the manner in which the trophoblast invades the uterine wall is similar to the invasive spread of neoplastic tissue. It is obviously essential to fetal well-being for the trophoblast to invade the uterine wall and establish vascular contact with the maternal circulatory system. Therefore, it is possible that just as various neoplastic growths can impede the rejection mechanisms of the host (Kamo and Friedman, 1978) some regulatory influences are brought to bear upon the maternal immune system during gestation. Superficially, it would appear unlikely that this suppressive effect is due to an immunosuppressive serum factor as several tumor-derived factors suppress predominantly IgM responses in contrast to the effect seen in pregnant mice (Kamo and Friedman, 1978; Watanabe et al., 1977). On the other hand it has been shown that pregnancy associated alpha-fetoprotein (AFP) does preferentially suppress thymusdependent IgG and IgA responses in vitro (Murgita, 1976; Murgita et al., 1978). In fact, pre-incubation of adult lymphocytes with 200/xg/ml AFP induced suppressor function in vitro by acting upon a specific subpopulation of Ly 1 + 2 - suppressor inducer T-lymphocytes. This non-specific suppressor cell has also been demonstrated in tumor systems (Watanabe et al., 1977). These facts are consistent with our demonstration of a reversible pregnancy associated non-specific defect of Tlymphocyte-dependent functions. There appeared to be no irreversible alteration in precursor cell frequencies or activity in vivo. Other investigators have shown that female mice (Carter, 1976; Fabris, 1973) and humans (Smith et al., 1972; Purtillo et al., 1972) show deficient cell-mediated

346 immune responses during pregnancy. The inability to mount a normal IgG and IgA response to thymus-dependent antigens may presumably be due to deficient T-cell helper function. The consequences of these defects could be an impaired ability to resist infections by viral agents during pregnancy (Eickhoff et al., 1961). At this time it is impossible to determine whether the predominant effect was: (a) a loss of T-helper cells, or helper cell function, (b) an increase in T-suppressor or macrophage suppressor function, or (c) an increase in immunosuppressive serum factors such as alpha-fetoprotein or steroid hormones. The fact that the IgM responses were essentially normal implied that the afferent loop of the immune system was intact and that the frequency and proliferative capacity of IgM antibody-forming precursor cells were not affected. Alternatively, IgM precursor lymphocytes may be relatively resistant to the putative suppressive effects of AFP and pregnancy hormones. Previous work in this laboratory has shown that the T- and B-cell proportions in the spleen and lymph nodes of pregnant mice (Baines et al., 1977a) and humans (Baines et al., 1977b) were relatively unaffected by pregnancy in spite of a marked thymic involution coincident with depressed humoral and cellular immunocompetence in both syngeneic and allogeneic pregnancy (Millar et al., 1973). Although some recent evidence supporting a role of suppressor T-cells in the regulation of the maternal response to paternal antigens in multiparous mice has been published (Clark et al., 1978, 1980; Beer et al., 1976; Suzuki and Tomasi, 1979; Chaouat et al., 1979, 1981), the way in which they affect general immunocompetence is still uncertain. It is possible that the suppressor cells directly release 'short-range' inhibitory lymphokines which non-specifically inhibit local lymphoproliferation and the generation of both IgM and IgG responses (Suzuki and Tomasi, 1979; Tomasi et al., 1977; Clark and McDermott, 1978). On the other hand, the systemic inhibitory effects of a first pregnancy are more likely to be due to a factor present in significant quantity such as AFP or the steroid hormones synthes!zed by the placenta. The suppression seen in these studies is probably due to a combination of both these effects. The reason that allogeneic pregnancy did not impair immunocompetence as markedly as syngeneic pregnancy remains unclear at this time. It is possible that natural allogeneic pregnancy provides an opportunity for the general augmentation of helper function via the 'allogeneic effect' phenomenon due to the stimulation of maternal T-cells by fetal cells. Therefore, immune responses could be modestly enhanced by allogeneic pregnancy, thereby concealing the underlying suppression evident during syngeneic pregnancy. In conclusion, the physiological and endocrinological alterations accompanying pregnancy are capable of mobilizing non-specific immunoregulatory mechanisms which impair thymus-dependent immune responses. The impairment does not alter either the numbers of specific precursor B-lymphocytes in the spleen or the time course of the response in vivo and appears to act at the helper or accessory cell level.

347

Acknowledgement This study was supported by Grant MA5271 from the Canadian Medical Res e a r c h C o u n c i l . T h e a u t h o r s w i s h t o a c k n o w l e d g e t h e t e c h n i c a l a s s i s t a n c e o f M s . I. L o u w m a n , M s . B. M i l g r o m a n d M s . E. Vizi a n d t h e s e c r e t a r i a l w o r k o f M s . L. Lacroix.

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