- Email: [email protected]
Response from Zinkernagel et al
C O M M E N T
antigen does not affect the long-term survival of these cells. In this situation, long-lived resting memory CD8 ÷ cells are able to cause efficient elimination of intravenously administered virus. However, if the virus is administered by other routes, for example into the brain or the peritoneal cavity, the response to the virus is appreciably slower than the rapid response seen with preactivated memory CD8 ÷cells. Based on these and other findings, the authors conclude that efficient secondary responses to pathogens entering the body at peripheral sites depend upon memory CD8 + cells being maintained as activated cells with direct cytotoxic T lymphocyte (CTL) activity, thus leading to an immediate response against the pathogen. According to this view, optimal long-term memory requires continuous exposure to residual depots of specific antigen. Contact with specific antigen is not needed for the survival of memory cells per se but is crucial for maintaining the direct CTL activity of these cells. In defending this view, the authors ascribe earlier claims of efficient secondary responses mediated by resting memory cells to the use of inadequate methods for assaying functional memory. The experiments of Bachmann et a l ) 9 are elegant and beautifully designed, and the data are, by and large, convincing. But have the authors made a fundamental point? As discussed above, their contention is that maintaining memory cells in an activated state is only crucial when pathogens enter the body at peripheral sites rather than via the bloodstream. With peripheral infection, pre-activated memory cells will mount an immediate response and thus promote rapid rejection of the pathogen concerned. By contrast, the response of resting memory cells will be preceded by a significant delay while these cells first recognize antigen on antigen-presenting cells (APCs) and then differentiate into CTLs. The key issue is whether this delay is physiologically relevant. The authors argue that the delay in the response of resting memory cells is particularly important when a pathogen is introduced directly into the brain, skin or the
peritoneal cavity. However, with the possible exception of the skin, it is questionable whether infection via these routes ever occurs naturally. Thus, in most situations pathogens enter the body via mucosal tissues and are carried to the draining lymph nodes (LN), thus coming into rapid contact with recirculating resting memory cells. As exposure to antigen in vitro causes resting memory cells to differentiate into CTL within 24 h (R. Ahmed, pers. commun.), only a brief period (12 days) is likely to be required for resting memory precursors in LN to begin their attack against the virus. A priori, one would expect this slight delay to be of little consequence, especially as the precursor frequency of resting memory cells is very high. However, for a highly pathogenic virus one might argue that even a very short delay in the response to the virus could be crucial. The counterargument is that if the host has survived initial infection with a virulent virus, it will inevitably survive secondary infection, which poses the troubling question of whether memory is really essential or merely represents an epiphenomenon! Despite the above comments, the notion that maintaining memory cells in a chronic state of activation contributes significantly to the intensity of the secondary response is clearly attractive and needs to be explored further. The data of Bachmann et al. 19 deal solely with CD8 ÷ cell memory, and in the future it will be important to seek parallel information on memory at the level of CD4 ÷ cells and B cells. For these cells, the additional consideration
Response from Zinkernagel et al. he critical review on T-cell T memory by Dr Sprent is a pleasure to read, not least because the notion of T-cell memory, which can protect against infections of the skin and other organs where antibodies cannot generally reach, is finally getting some recognition 1,2. Our data illustrate that cytotoxic T cells must be activated by some form of anti-
Copyright © 1997 Elsevier Science Ltd. All rights reserved. 0966 842X/97/$17.00
TRENDS IN MICROBIOLOGY
260
VOL. S
NO.
(now excluded for CD8 ÷ cells) that contact with persisting antigen may be important for keeping memdry cells alive still remains. Thus, if memory CD4 +cells are intrinsically longlived, why do we have to be repeatedly boosted with tetanus toxoid? Such questions indicate that there is still life in the study of memory.
References 1 Sprent,J. (1994) Cell 76, 315-322 2 Zinkernagel, R.M. et al. (1996) Annu. Rev. Immunol. 14, 333-368 3 Ahmed, R. and Gray, D. (1996) Science 272, 54-60 4 Gray, D. etal. (1996)Immunol. Rev. 150, 45-61 5 Sprent,J. Curr. Opin. lmmunol. (in press) 6 Gowans,J.L. and Uhr, J.W. (1966) J. Exp. Med. 124, 1017-1030 7 Sprent,J. and Miller, J.F.A.P. (1976) Cell. Immunol. 21,314-326 8 Gray, D. and Skarvall, H. (1988) Nature 336, 70-72 9 Gray, D. and Matzinger, P. (1991) J. Exp. Med. 174, 969-974 10 Oehen, S. et al. (1992)J. Exp. Med. 176, 1273-1281 11 Mullbacher,A. (1994)J. Exp. Med. 179, 317-321 12 Lau, L.L. et al. (1994) Nature 369, 648-652 13 Hou, S. et al. (1994) Nature 369, 652-654 14 Bruno, L., Kirberg,J. and von Boehmer,H. (1995) Immunity 2, 37-43 15 Beverley,P.C.L. (1990) ImmunoL Today 11,203-205 16 Tough, D.F., Borrow, P. and Sprent,J. (1996) Science 272, 1947-1950 17 Kundig, T.M. et al. (1996)Proc. Natl. Acad. Sci. U. S. A. 93, 9716-9723 18 Kundig, T.M. et al. (1996) Immunol. Rev. 150, 63-90 19 Bachmann, M.F. et al. (1997) Proc. Natl. Acad. Sci. U. S. A. 94, 640-645
gen to be ready and recruitable immediately after infection. Dr Sprent questions the relevance of this antigen dependence by arguing that such infections do not occur naturally and that antibodies take care of reinfections on mucosal surfaces or reinfections spreading via the bloodstream to the spleen and other organs. However, in this key point, we do not agree. Reinfections in solid peripheral organs generally occur from lowPII: S0966-842X(97)01074-3
7
3UL
1997
C O M M E N T
level persistent infections [as shown for cytopathic viruses (for example, herpes), non-cytopathic viruses (for example, hepatitis B virus), lymphocytic choriomeningitis virus (LCMV) and several others]. Immune surveillance is dependent on both effector T cells, for local control, and neutralizing antibodies against hematogenic spread of the virus. Interestingly, the extension of this concept to CD4 ÷ T helper cells was analyzed several years ago and was one of our first key observations in support of the notion that only activated T cells can maintain efficient T-cell memory3. Mice immunized with one of the two major serotypes of vesicular stomatitis virus (VSV) induce both crossreactive cytotoxic T cells (as well as crossreactive T helper cells) and, by definition, specific neutralizing B cells that do not crossreact. We asked whether the priming of T help with one serotype enhances the T-helpdependent neutralizing immunoglobulin G (IgG) response against the other serotype on reinfection. We showed that, despite primed and functional crossreactive T help, an enhancement of the neutralizing B-cell response against the second serotype beyond that of a primary response could be demonstrated only transiently: for no later than 7-14days after priming. Similar findings have subsequently been reported for influenza-specific antibody responses4, which fits with the clinical experience that immunity to a particular influenza A virus does not significantly improve resistance against a distinct A virus. These experiments also seem to indicate that it is the activation status and the precursor frequency of switchable B cells, rather than the frequency of T help, that is limiting against viruses. Dr Sprent discusses the important general aspects of antibody versus T-cell memory in a broader evolutionary context, but we would like to see this concept interpreted in a more clear-cut way. We have attempted to summarize these general aspects in our recent reviews>V; generally, cytopathic viruses are controlled during primary and secondary infections via soluble mediators, including interleukins and
TRENDS
neutralizing antibodies, whereas non-cytopathic viruses are controlled by perforin-dependent CD8 ÷ T cells in primary infections and by cytotoxic T cells plus antibodies during secondary infectionss,8. We earlier argued s,6 that a host surviving a primary infection is not fundamentally dependent upon memory, although it improves fitness. In contrast, during the physiological phase of immunodeficiency, in the eggs of birds or fish and after birth in mammals, memory is crucial. A physiological phase of immunodeficiency is therefore rendered possible and is necessary because major histocompatibility complex (MHC)-polymorphism, which is a direct consequence of MHCrestricted T-cell recognition, causes allorecognition and graft rejection between individuals of a species, as well as between mother and offspring. This necessitates immunosuppression of the mother during pregnancy and a physiological state of immunoincompetence in the offspring before birth. Therefore, soluble, transferable, protective memory antibodies are essential for the survival of the offspring during the period of physiological immunodeficiency (several weeks for mice and several months for humans). As only antibody memory is transferable, B-cell memory is crucial for species survival, whereas the nontransferable T-cell memory is not. Of course, immunity also contributes to overall fitness. As pregnant mothers must not be endangered by infections and as not all epidemiologically relevant infections are encountered during the 3 and 40 weeks of pregnancy of mice and humans, respectively, to generate transmissible antibody immunity, immunological memory must be maintained for long periods of time. This was classically attributed to a 'special memory quality'. Therefore, it is not surprising, perhaps, that our experiments on the role of antigens in maintaining T-cell memory, including the short life of functional T helper cell memory and protective cytotoxic T cell memory against peripheral viral infections, have met with tremendous resistance from the immunological community. Nevertheless,
IN MI(;R('BI()I.()(;Y
261
vo,_.
5
No.
several experiments have clearly indicated that antigen depots or reinfection are necessary for keeping B cells activated to differentiate into mature plasma cells producing antibodies that carry protective transmissible memory9-11. We are confident about the concept that biologically relevant (i.e. protective) immunological memory is antigen-dependent. This notion is not only simple but also overcomes the need for a special memory quality and makes sense biologically; obviously, it also impinges on the rationales behind preventive vaccination and adoptive immunotherapy. Rolf M. Zinkernagel, Martin F. Bachmann, Thomas M. Kiindig and Hans Hengartner Dept of Pathology, Institut fiir Experimentelle Immunologie, Universit~it Ziirich, University Hospital, CH-8091 Ziirich, Switzerland References
1 Bachmann,M.F.et al. (1997)Proc. Natl. Acad. Sci. U. S. A. 94, 640-645 2 Kiindig,T.M. et al. (1996)Proc. Natl. Acad. Sci. U. S. A. 93, 9716-9723 3 Roost,H-P.,Charan,S. and Zinkernagel,R.M. (1990)Eur. J. Immunol. 20, 2547-2554 4 Liang,S. et al. (1994)J. Immunol. 152, 1653-1661 5 Zinkernagel,R.M. (1996)Science 271, 173-178 6 Zinkemagel,R.M.et aI. (1996)Annu. Rev. Immunol. 14, 333-368 7 Kiindig,T.M. et al. (1996)Immunol. Rev. 150, 63-90 8 K~igi,D. etal. (1996)Annu.Rev. Immunol. 14, 207-232 9 Gray,D. and Skarvall,H. (1988)Nature 336, 70-72 10 Thorbecke,G.J. (1990)J. Immunol. 145, 2779-2790
11 Bachmann,M.F.et al. (1996)J. Exp. Med. 183, 2259-2269
Students! Subscribe to TIM today at half price. Use the bound-in card.
7
JULY
1997
antigen does not affect the long-term survival of these cells. In this situation, long-lived resting memory CD8 ÷ cells are able to cause efficient elimination of intravenously administered virus. However, if the virus is administered by other routes, for example into the brain or the peritoneal cavity, the response to the virus is appreciably slower than the rapid response seen with preactivated memory CD8 ÷cells. Based on these and other findings, the authors conclude that efficient secondary responses to pathogens entering the body at peripheral sites depend upon memory CD8 + cells being maintained as activated cells with direct cytotoxic T lymphocyte (CTL) activity, thus leading to an immediate response against the pathogen. According to this view, optimal long-term memory requires continuous exposure to residual depots of specific antigen. Contact with specific antigen is not needed for the survival of memory cells per se but is crucial for maintaining the direct CTL activity of these cells. In defending this view, the authors ascribe earlier claims of efficient secondary responses mediated by resting memory cells to the use of inadequate methods for assaying functional memory. The experiments of Bachmann et a l ) 9 are elegant and beautifully designed, and the data are, by and large, convincing. But have the authors made a fundamental point? As discussed above, their contention is that maintaining memory cells in an activated state is only crucial when pathogens enter the body at peripheral sites rather than via the bloodstream. With peripheral infection, pre-activated memory cells will mount an immediate response and thus promote rapid rejection of the pathogen concerned. By contrast, the response of resting memory cells will be preceded by a significant delay while these cells first recognize antigen on antigen-presenting cells (APCs) and then differentiate into CTLs. The key issue is whether this delay is physiologically relevant. The authors argue that the delay in the response of resting memory cells is particularly important when a pathogen is introduced directly into the brain, skin or the
peritoneal cavity. However, with the possible exception of the skin, it is questionable whether infection via these routes ever occurs naturally. Thus, in most situations pathogens enter the body via mucosal tissues and are carried to the draining lymph nodes (LN), thus coming into rapid contact with recirculating resting memory cells. As exposure to antigen in vitro causes resting memory cells to differentiate into CTL within 24 h (R. Ahmed, pers. commun.), only a brief period (12 days) is likely to be required for resting memory precursors in LN to begin their attack against the virus. A priori, one would expect this slight delay to be of little consequence, especially as the precursor frequency of resting memory cells is very high. However, for a highly pathogenic virus one might argue that even a very short delay in the response to the virus could be crucial. The counterargument is that if the host has survived initial infection with a virulent virus, it will inevitably survive secondary infection, which poses the troubling question of whether memory is really essential or merely represents an epiphenomenon! Despite the above comments, the notion that maintaining memory cells in a chronic state of activation contributes significantly to the intensity of the secondary response is clearly attractive and needs to be explored further. The data of Bachmann et al. 19 deal solely with CD8 ÷ cell memory, and in the future it will be important to seek parallel information on memory at the level of CD4 ÷ cells and B cells. For these cells, the additional consideration
Response from Zinkernagel et al. he critical review on T-cell T memory by Dr Sprent is a pleasure to read, not least because the notion of T-cell memory, which can protect against infections of the skin and other organs where antibodies cannot generally reach, is finally getting some recognition 1,2. Our data illustrate that cytotoxic T cells must be activated by some form of anti-
Copyright © 1997 Elsevier Science Ltd. All rights reserved. 0966 842X/97/$17.00
TRENDS IN MICROBIOLOGY
260
VOL. S
NO.
(now excluded for CD8 ÷ cells) that contact with persisting antigen may be important for keeping memdry cells alive still remains. Thus, if memory CD4 +cells are intrinsically longlived, why do we have to be repeatedly boosted with tetanus toxoid? Such questions indicate that there is still life in the study of memory.
References 1 Sprent,J. (1994) Cell 76, 315-322 2 Zinkernagel, R.M. et al. (1996) Annu. Rev. Immunol. 14, 333-368 3 Ahmed, R. and Gray, D. (1996) Science 272, 54-60 4 Gray, D. etal. (1996)Immunol. Rev. 150, 45-61 5 Sprent,J. Curr. Opin. lmmunol. (in press) 6 Gowans,J.L. and Uhr, J.W. (1966) J. Exp. Med. 124, 1017-1030 7 Sprent,J. and Miller, J.F.A.P. (1976) Cell. Immunol. 21,314-326 8 Gray, D. and Skarvall, H. (1988) Nature 336, 70-72 9 Gray, D. and Matzinger, P. (1991) J. Exp. Med. 174, 969-974 10 Oehen, S. et al. (1992)J. Exp. Med. 176, 1273-1281 11 Mullbacher,A. (1994)J. Exp. Med. 179, 317-321 12 Lau, L.L. et al. (1994) Nature 369, 648-652 13 Hou, S. et al. (1994) Nature 369, 652-654 14 Bruno, L., Kirberg,J. and von Boehmer,H. (1995) Immunity 2, 37-43 15 Beverley,P.C.L. (1990) ImmunoL Today 11,203-205 16 Tough, D.F., Borrow, P. and Sprent,J. (1996) Science 272, 1947-1950 17 Kundig, T.M. et al. (1996)Proc. Natl. Acad. Sci. U. S. A. 93, 9716-9723 18 Kundig, T.M. et al. (1996) Immunol. Rev. 150, 63-90 19 Bachmann, M.F. et al. (1997) Proc. Natl. Acad. Sci. U. S. A. 94, 640-645
gen to be ready and recruitable immediately after infection. Dr Sprent questions the relevance of this antigen dependence by arguing that such infections do not occur naturally and that antibodies take care of reinfections on mucosal surfaces or reinfections spreading via the bloodstream to the spleen and other organs. However, in this key point, we do not agree. Reinfections in solid peripheral organs generally occur from lowPII: S0966-842X(97)01074-3
7
3UL
1997
C O M M E N T
level persistent infections [as shown for cytopathic viruses (for example, herpes), non-cytopathic viruses (for example, hepatitis B virus), lymphocytic choriomeningitis virus (LCMV) and several others]. Immune surveillance is dependent on both effector T cells, for local control, and neutralizing antibodies against hematogenic spread of the virus. Interestingly, the extension of this concept to CD4 ÷ T helper cells was analyzed several years ago and was one of our first key observations in support of the notion that only activated T cells can maintain efficient T-cell memory3. Mice immunized with one of the two major serotypes of vesicular stomatitis virus (VSV) induce both crossreactive cytotoxic T cells (as well as crossreactive T helper cells) and, by definition, specific neutralizing B cells that do not crossreact. We asked whether the priming of T help with one serotype enhances the T-helpdependent neutralizing immunoglobulin G (IgG) response against the other serotype on reinfection. We showed that, despite primed and functional crossreactive T help, an enhancement of the neutralizing B-cell response against the second serotype beyond that of a primary response could be demonstrated only transiently: for no later than 7-14days after priming. Similar findings have subsequently been reported for influenza-specific antibody responses4, which fits with the clinical experience that immunity to a particular influenza A virus does not significantly improve resistance against a distinct A virus. These experiments also seem to indicate that it is the activation status and the precursor frequency of switchable B cells, rather than the frequency of T help, that is limiting against viruses. Dr Sprent discusses the important general aspects of antibody versus T-cell memory in a broader evolutionary context, but we would like to see this concept interpreted in a more clear-cut way. We have attempted to summarize these general aspects in our recent reviews>V; generally, cytopathic viruses are controlled during primary and secondary infections via soluble mediators, including interleukins and
TRENDS
neutralizing antibodies, whereas non-cytopathic viruses are controlled by perforin-dependent CD8 ÷ T cells in primary infections and by cytotoxic T cells plus antibodies during secondary infectionss,8. We earlier argued s,6 that a host surviving a primary infection is not fundamentally dependent upon memory, although it improves fitness. In contrast, during the physiological phase of immunodeficiency, in the eggs of birds or fish and after birth in mammals, memory is crucial. A physiological phase of immunodeficiency is therefore rendered possible and is necessary because major histocompatibility complex (MHC)-polymorphism, which is a direct consequence of MHCrestricted T-cell recognition, causes allorecognition and graft rejection between individuals of a species, as well as between mother and offspring. This necessitates immunosuppression of the mother during pregnancy and a physiological state of immunoincompetence in the offspring before birth. Therefore, soluble, transferable, protective memory antibodies are essential for the survival of the offspring during the period of physiological immunodeficiency (several weeks for mice and several months for humans). As only antibody memory is transferable, B-cell memory is crucial for species survival, whereas the nontransferable T-cell memory is not. Of course, immunity also contributes to overall fitness. As pregnant mothers must not be endangered by infections and as not all epidemiologically relevant infections are encountered during the 3 and 40 weeks of pregnancy of mice and humans, respectively, to generate transmissible antibody immunity, immunological memory must be maintained for long periods of time. This was classically attributed to a 'special memory quality'. Therefore, it is not surprising, perhaps, that our experiments on the role of antigens in maintaining T-cell memory, including the short life of functional T helper cell memory and protective cytotoxic T cell memory against peripheral viral infections, have met with tremendous resistance from the immunological community. Nevertheless,
IN MI(;R('BI()I.()(;Y
261
vo,_.
5
No.
several experiments have clearly indicated that antigen depots or reinfection are necessary for keeping B cells activated to differentiate into mature plasma cells producing antibodies that carry protective transmissible memory9-11. We are confident about the concept that biologically relevant (i.e. protective) immunological memory is antigen-dependent. This notion is not only simple but also overcomes the need for a special memory quality and makes sense biologically; obviously, it also impinges on the rationales behind preventive vaccination and adoptive immunotherapy. Rolf M. Zinkernagel, Martin F. Bachmann, Thomas M. Kiindig and Hans Hengartner Dept of Pathology, Institut fiir Experimentelle Immunologie, Universit~it Ziirich, University Hospital, CH-8091 Ziirich, Switzerland References
1 Bachmann,M.F.et al. (1997)Proc. Natl. Acad. Sci. U. S. A. 94, 640-645 2 Kiindig,T.M. et al. (1996)Proc. Natl. Acad. Sci. U. S. A. 93, 9716-9723 3 Roost,H-P.,Charan,S. and Zinkernagel,R.M. (1990)Eur. J. Immunol. 20, 2547-2554 4 Liang,S. et al. (1994)J. Immunol. 152, 1653-1661 5 Zinkernagel,R.M. (1996)Science 271, 173-178 6 Zinkemagel,R.M.et aI. (1996)Annu. Rev. Immunol. 14, 333-368 7 Kiindig,T.M. et al. (1996)Immunol. Rev. 150, 63-90 8 K~igi,D. etal. (1996)Annu.Rev. Immunol. 14, 207-232 9 Gray,D. and Skarvall,H. (1988)Nature 336, 70-72 10 Thorbecke,G.J. (1990)J. Immunol. 145, 2779-2790
11 Bachmann,M.F.et al. (1996)J. Exp. Med. 183, 2259-2269
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7
JULY
1997