Response from Slifka and Ahmed

COMMENT recently proposed for all capsule and lipopolysaccharide genes in Gram-negative bacteria H. As the complete O antigen/capsule region has now been sequenced and the respective structures have been determined, mutagenesis of the individual ORFs in conjuction with chemical analysis studies should permit the characterization of the biosynthetic pathway for V. cholerae 0139-specific O antigen and capsular polysaccharides. Acknowledgements This work has been supported by grants from the Australian Research Council, the Clive and Vera Ramaciotti Foundations and the committee for Diarrhoeal Diseases Research of the Global Vaccine Program of the World Health Organization. Uwe H. Stroeher and Paul A. Manning Microbial Pathogenesis Unit, Dept of Microbiology and Immunology, University of Adelaide, Adelaide, SA 5005, Australia References 1 Mooi, F.R. and Bik, E.M. {1997) Trends Microbiol. 5, 161-165 2 Manning, P.A. et al. {1986} Infect. lmmun. 53,272-277 3 Ward, H.M. etal. {1987) Gene 55, 197-204 4 Manning, P.A., Stroeher, U.H. and Morona, R. {1994)in Vibrlo cholerae and Cholera - Molecular to Global Perspectwes IWachsmuth, I.K., Blake, P.A. and Olsvik, O., eds), pp. 77-94, ASM Press B cell longevity and immunological memory their recent review in this journal ~, Iofnwhich hypothesizes that the longer W immunological memory may be ascribed to B cell longevity, Mark Slifka and Raft Ahmed make several points of clinical interest. They describe the trafficking of mature plasma cells from lymph nodes into the bone marrow. This is compatible with the observation that mature plasma cells are found in normal marrow" in larger numbers in adults than in children. Furthermore, it is known that bone marrow plasma cells express large amounts of bcl-2 and are relatively resistant to radiotherapy. These features indicate that these cells may be capable of a greater longevity than plasma cells elsewhere. However, the authors' hypothesis does not concur with published data in several significant ways: first, the longevity of an immune response requires the plasma cells to have a long intermitotic lifespan. This feature has not been demonstrated for B cells in the bone marrow. Indeed, measures of B cell intermitotic time using a range of different techniques have all indicated that they have a wide range of considerably shorter intermitotic lifespans than T cells, and intermitotic times may be determined by factors regulating the total numbers of lymphocytes, for example competition for antigen and T cell help 2. Second, both maintenance antibody levels and B cell 'memory' in animal models often appear to be dependent on the persistence of antigen ~. Other models, such as memory imprinting, have been proposed to explain systems in which persistent antigen has not been identified ~. In either situation, persistence of individual cells has not been a significant factor. Third, there are clear clinical data showing that with age, humans and rodents develop progressively less diverse antibody repertoires. There is a steady increase in monoclonal immunoglobulins with age, particularly of the immunoglobulin M (IgM) subclass. A large proportion of these are autoreactive. If the Slifka hypothesis is correct, we would expect an increased diversity of polyclonal immunoglobulin G (lgG) antibody responses to non-self antigens with time, which is contrary to published findings. As there is evidence for longevity in some T cell populations ~, and of some antigen depots, it seems debatable whether long-lived B cells are significant in the maintenance of immunological memory to viruses, 5 Bik, E.M. etal. {1995)EMBO]. 14, 209-216 6 Comstock, L.E. et al. (1995) Infect. Immun. 63,317-323 7 Stroeher, U.H. et al. (1995) Proc. Natl. Acad. Sci. U. S. A. 92, 10374-10378 8 Stroeher, U.H. et al. (1995) Gene 155, 67-72 9 Fallarino, A. et al. J. BacterioI. (in press) 10 Bik, E.M. et al. (1996)Mol. Microbiol. 20, 799-811 11 Waldor, M.K., Colwell, R. and Mekalanos, J.J. (1994)Proc. Natl. Acad. Sci. U. S. A. 91, 11388-11392 12 Stroeher, U.H. et al. J. Bacteriol. {in press) 13 Comstock, L.E. et al. (1996) Mol. Microbiol. 19, 815-826 14 Reeves, P.R. et al. (1996) Trends Microbiol. 12,495-503 particularly without direct evidence of their existence. Colin A. Michie Dept of Paediatrics, Ealing Hospital NHS Trust, Uxbridge Road, Middlesex, UK UBI 3HW Irvin A. Lampert Dept of Histopathology, Royal Postgraduate Medical School, Du Cane Road, London, UK W 12 0NN References 1 Slifka, M.K. and Ahmed, R. (1996) Trends Microbiol. 4, 394-400 2 Freitas, A. and Rocha, B.B. (1993) Immunol. Today 14, 25-28 3 Gray, D. (1993)Trends Microbiol. 1, 39-42 4 UytdeHaag, F., Van der Heilden, R. and Osterhaus, A. {1991) IrnmunoI. Today 12, 439-444 5 McLean, A.R. and Michie, C.A. (1995) Proc. Natl. Acad. Sci. U. S. A. 92, 3707-3711 Response from Slifka and Ahmed he comments by Michie and T Lampert concerning our recent review are much appreciated. We all agree that the bone marrow is a major anatomical site of plasma cell localization and that plasma cells continue to accumulate in the bone marrow as we age ~. It is true that the vast majority of B cells in the bone marrow are very short-lived, as would be expected in an organ of continual B cell lymphopoiesis, but some observations have indicated that longlived B cells and T cells can be found in the bone marrow compartment >4. These studies show that the bone marrow is not devoid of long-lived cells, suggesting that other long-lived cell types (such as plasma cells) may also reside in this anatomical site. The role of persisting antigen in maintaining B cell and T cell memory is an issue that is not fully resolved. It is possible that different subpopulations COMMENT of B and T cells may have differing requirements for antigen or antigen-antibody complexes. In this respect, it is important to note that there are probably several mechanisms involved with maintaining prolonged antibody production and that the hypothesis of plasma cells having an extended life span is not exclusive to these other more-conventional models of antibody production. It is possible that long-lived plasma cells may co-exist in a system governed by intermittent or continual antigenic stimulation/differentiation of memory B cells into antibody-secreting cells. In this case, plasma cells with a longer life span would only lower the number of times that memory B cells would have to divide/differentiate in order to sustain an ongoing humoral immune response. Furthermore, as mature plasma cells no longer express surfacebound immunoglobulin receptors" or major histocompatibility complex (MHC) class II molecules% it is unlikely that further antigenic stimulation or direct CD4* T cell-plasma cell interactions will have a major impact on plasma cell survival. With age, monoclonal gammopathies arise in animals: and in humans 8, but this appears to be caused by a disregulated outgrowth of monoclonal B cell clones ~ and has little to do with the diversity of the antibody repertoire, which remains largely intact in the aged I°. it is not known if long-lived plasma cells play a role in these monoclonal disorders, and the effects of long-lived plasma cells on the diversity of polyclonal immunoglobulin G (IgG) responses have also yet to be determined. The aim of our review was to draw attention to plasma cells and their respective role in the maintenance of lnng-term antibody production. We believe that this has been a long-overlooked component of humoral immunity and that there is much yet to be learned about these terminally differentiated antibody-secreting cells. Mark K. Slifka Dept of Immunology, IMM- 16, The Scripps Research Institute, 10666 North Torrey Pines Roa& La Jolla, CA 92037, USA The role of p53 in virally associated tumors he role of p53 in human cancer has been exhaustively discussed T and reviewed during recent years; its central role in this disease certainly justifies these efforts. However, the role of p53 in virus replication has received relatively little attention. The recent review in this journal by James Nell and colleagues I is therefore especially welcome: after all, p53 was first identified through its interactions with viral antigens. Seventeen years later, we and others are trying to use our knowledge of p53 and its role in virus replication to devise new cancer therapeutics ~. When DNA tumor viruses infect resting cells, they force them into S phase. They inactivate p53, suppress apoptosis and render cells less visible to immune surveillance. These very same steps occur in human cancers. Most, if not all, cancers have defects in the Rb cell cycle checkpoint: these defects allow uncontrolled entry into S phase s, and the majority contain defects in p53 (Ref. 4). Antigen presentation is often suppressed in cancer cells by one of several mechanisms that resemble approaches used by DNA viruses ~. This amazing convergence is not coincidental; DNA viruses need to drive quiescent cells TI~.t N I ) S I N N I l ( into S phase for efficient replication ot their own DNA, just as tumor cells need to duplicate their DNA under conditions in which this process would be forbidden. A consequence of uncontrolled entry into S phase, whether induced by viruses or, in cancer cells, by mutations in the Rb cell cycle checkpoint, is increased apoptosis. This penalty is avoided by mutation of p53. This is a simplistic view of the role of p53 in human cancer and virus replication. On closer inspection, there is much to learn about both. For example, which function of p53 is most important in human cancer? Loss of p53 function in cancer cells leads to decreased apoptosis but also increases genome instability, decreases production of thrombospondin (a protein that inhibits the growth of vascular endothelial cells) and decreases production of the cell-cycle regulator p21. Any one of these events could confer a selective advantage to a tumor cell. In viral infections, as Nell and colleagues ~ discuss, it may be necessary to suppress apoptosis caused by forced entry into S phase. But other viral proteins could achieve this effect: for example, adenovirus E 1B 19K I~.{)BI()I {}(;~ 181 V{)l. "~ N{)..S Raft Ahmed Emory Vaccine Center and the Dept of Microbiology and Immunology, Emory University School of Medicine, 1510 Clifton Road, Atlanta, GA 30307, USA References 1 Benner, R., Hijmans, W. and Haaijman, J.J. (1981) Clin. Exp. Immunol. 46, 1-8 2 Ropke, C. and Everett, N.B. {1973) Cell Tissue Kinet. & 499 3 Claesson, M.H., Ropke, C. and Hougen, H.P. (1974) Stand. J. lmmunol. 3, 597-604 4 Ropke, C., Hougen, H.P. and Everett, N.B. (1975) Cell. lmmunol. 15, 82-93 5 Abney, E.R. etal. (1978)J. lmmunol. 120, 2041-2049 6 Halper, J. etal. {1978)J. lmmunol. 120, 1480-1484 7 Radl, J. (1990} lmmunol. Today 11, 234-236 8 Ligtham G.J. et el. {1990}Mech. Ageing De:. 52,235-243 9 Stall, A.M. etal. (1988) Proc. Natl. Acad. Sci. U. S. A. 85, 7312-7316 10 Zharhary, D. and Klinman, N.R. (1984) .1. lmmunoL 133, 2285-2287 suppresses most types of apoptosis. Perhaps p53 has other functions that DNA viruses need to eliminate. This should be investigated thoroughly. Further investigation of the precise role of p53 in viral replication could have two major benefits. First, it may continue to elucidate the precise functions of p53 in normal cells and in cancer. Second, it could lead to new therapeutic strategies to attack cancer cells specifically. We have already shown that a virus that fails to inactivate p53 is restricted in host range to cells lacking functional p53 (i.e. cancer cells), and we are attempting to use this virus as a therapeutic agent 2. A better understanding of the role of p53 in virus replication should allow further development of this concept to improve efficacy (by killing cancer cells more efficiently) and safety (by further restricting growth of these viruses in normal cells). Other viruses that interact with p53 could also be used as therapeutic agents as their biology is unravelled and the function of p53 in their growth is clarified. All of these efforts may indeed help us catch the guardian off-guard, and may ultimately bring benefit to patients suffering from the most difficult kinds of tumors to treat: those that lack the p53 guardian. 1X4AY 1997