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How does the helper T cell activate the resting B cell when it recognizes antigen on the B-cell surface?
(~) INSTITUT P.,',"3TEUR/ELSEVIER
Res. lmmunol. IBt% 19:,,.,, 141,405-454
Paris 1990
32nd FORUM IN IMMUNOLOGY
H O W D O E S T H E H E L P E R T C E L L ACTIVATE T H E R E S T I N G B C E L L W H E N liT R E C O G N I Z E S A N T I G E N ON T H E B - C E L L S U R F A C E ?
Organized by D.C. Parker
INTRODUCTION
The resting B lymphocyte is all potential, a container for a unique set of antibody genes created by DNA recombination in the bone marrow. To express its potential as secreted antibody, the B cell must be activated to proliferate and differentiate into a clone of antibody-secreting cells. Activation depends upon binding of antigen to membrane immunoglobulin (lg) on the B cell. The result is a specific antibody response by the process of clonal selection, since the membrane Ig is a sample of the antibody that the clone will secrete. This simple picture is complicated by the fact that a normal antibody response to a protein antig, en requires the participation of helper T lymphocytes specific for the same antigen particle. The question of how T cells help B cells has kept a number of immunologists occupied for two decades. Con-
siderable progress has bee.~ mad.e with this problem, which reveals new leve!s of complexity as it unfolds. The new consensus holds that B cells get help by acting as antigen-specific antigen-presenting cells, an idea that was proposed in the mid-seventies as soon as the outlines of how MHC antigens guide T-cell recognition on cell surfaces became apparent (Sprent, 1978; Benacerraf, 1978), but was not directly demonstrated until nearly a decade later (Tony and Parker, 1985; reviewed in Chestnut and Grey, 1986). This requirement for direct cellular interaction between an antigen-presenting B cell and a helper T cell explains MHC-restriction of T-cell help. Since B lymphocytes can recognize native proteins and then process them for presentation, it also explains how B cells can make anti
406
32 nd F O R U M I N I M M U N O L O G Y
bodies that show exquisite specificity for protein conformation while receiving help from T cells that recognize small peptides. What remains to be resolved is the mechanism of help" which molecules and activation pathways does the helper T cell use to drive the B cell into proliferation and Ig secretion? That is the question addressed by the scientists who have contributed to this Forum. As noted in the comments by some of the centributors, this Forum produced considerable agreement but not consensus on a number of important issues. !) The role of membrane Ig, the antigen receptor of the B cell, is largely to enable antigen-specific antigen presentation. A signal from membrane Ig is not require.d for a vigorous antibody response, since the helper T cell can do the whole job once antigen is displayed on the B-cell surface in the context of MHC molecules. This result, although anticipated by some (Augustin and Coutinho, 19~0), is somewhat surprising in view of the well-documented signals that can be transmitted to B cells by cross-linking membrane lg. In fact, these signals may be more relevant to thymus-independent, anti-polysaccharide antibody respoe,~es. A membrane Ig signal can enhance a suboptimal T/B ." .
.
i
.
.
.
i. -~.
.
.
l l l t g ; l iat~tll,J l l ,
.
.
!,., _
___
pg;I l l i i p ~
L _ _
uy
increasing
cla~s ii expression, and may play a role in thymus-dependent responses in vivo, but in our own in vitro experiments with Th2 helper T-cell lines, we found no influence of membrane lg signalling on either the efficiency of antigen presentation by small B cells or their response to T-cell help (Tony et al., 1985). 2) Lymphokines acting in combination play an essential role, but the identified lymphokines by themselves are inadequate to induce clonal expansion and secretion by resting B cells. Most of us believe that some sort of cell contact is also required (although Julius points out that IL-5 appears to induce immunoglobulin secretion without proliferation in at least a proportion of small B cells, and Theze and Diu are actively pursuing a soluble factor that induces proliferation and secretion of resting B cells in their hands). In some
experiments, cell contact alone can induce at least the first round of DNA synthesis, while in other experiments, contact plus IL-4 or IL-2 is required. 3) Even in responses that are MHCrestricted at the T cell/B cell level, MHC restriction and antigen specificity have more to do with induction of helper activity by the B cell acting as an antigenpresenting cell than with the delivery of help to the B cell. This conclusion is based on contact-dependent activation of bystander small B cells (Whalen et al., 1988) as well as B-cell responses induced by T-cell lines and normal T cells activated by antibodies to the T-cell receptor complex in the absence of antigen or appropriate MHC alleles, as described in this Forum by Lipsky, Marshall and Noelle, Owens, Kehry and colleagues, Snow and colleagues, and Crow and Friedman. Swain and Croft point out that these responses are efficient only when the T cells are strongly activated, and so may represent only a minimal model for the delivery of help. Nevertheless, this result implies that the membrane molecules that provide the contact signals for help are distinct in part from those that activate the T ~ e i"l . In particular, if class I1 MHC molecules are involved in contact signal transduction, they would not need to be engaged ~,~r,|¢;,-,,,~ll,,
~ i ~ l
11~11~
h,, O~
~h,~ til~
T
~11
I "~ll
~ ; ~
~lltl~ii
receptor. 4) S~me component of the contact help machinery is inducible on the time scale of lymphokine synthesis and secretion, implying that help depends on induced synthesis or protein synthesisdependent modification of a membrane molecule on the helper T cell. The strongest statements on this theme are made by Noelle and colleagues, using activated, fixed helper T cells and by Kehry and colleagues using whole membranes of activated helper T cell~. Identification of this molecule would seem to be a top priority. By analogy with T-cell activation, it seems likely that multiple molecules will be involved in B-cell activation, including L F A - I / I C A M - I , CD2/LFA-3, CD45, and possibly CD4/class II. Other candidate molecules could be proposed. Antibody inhibition experiments are likely to be
HOW
T CELLS HELP B CELLS
informative, particularly if done with Fab fragments to minimize negative signalling through membrane molecules, but have not yet produced consistent results among the authors in this Forum. Looking further ahead, we can see that an adequate understanding of T-dependent B lymphocyte activation will require a description of how multiple external signals result in an integrated cellular response. The papers by Crow and Friedman, Snow and colleagues, and our own unpublished results suggest that contact help probably involves second messenger systems that are different from those induced by membrane Ig crosslinking. Unravelling causal connections among external signals, second messenger systems, and the cellular response will be difficult, because the biochemical changes occur in minutes, while T-cell help is a protract-
407
ed process, requiring up to 24 hours before most of the B cells are committed to a round of DNA synthesis, even in the presence of exogenous lymphokines. For optimal proliferation and differentiation to antibody secretion, T-cell help may need to be continuous over days (Owens). Therefore, as anticipated in some of the experiments described in this Forum, the B-cell response will need to be studied in stages (early events, stages of the cell cycle, and differentiadon ~o Ig secretion), and the requirements for passage from one stage to the ne~t studied individually. D.C. Parker Department of Molecular Genetics and Microbiology, Utfiversity of Massachusetts Medical School, Worcester, MA 01655 (USA)
References. At~t~t~sx~N,A.A. & Coux~Nuo, A. (1980), Specific T helper cells that activate B cells polyclonally. in vitro enrichment and cooperative function. J. exp. Med., 151,587. BENACERRAF,B. (1978), A hypothesis to relate the specificity of T lymphocytes and the activity of I region-specific IR genes in macrophages and B lymphocytes. J. Immunol., i20, i809. CHESNUT,R.W. & H.M. GREY(1986), Antigen presentation by B cells and its significance in T-B interactions. 4dvanc. Immunol., 39, 51. SeReNe, J. (1978), Restricted helper function of F~ hybrid T cells positively selected to heterologous erythrocytes in irradiated parental strain mice. - - II. Evidence for restrictions affecting helper cell induction and B-cell collaboration, both mapping to the K end of the H-2 complex. J. exp. Med., 147, 1159. TONY, H.-P. & PARKER,D.C. (1985), Major histocompatibility .co.mplex-restricted, polyclonal B-cell responses resulting from helper T-cell recognmon of anti-immunoglobulin presented by small B lymphocytes. J. exp. Med., 161, 223. ToNy, H.-P., PHIl.tiPS, N.E. & PARKE~, D.C. (1985), Role of membrane immunoglobulin crosslinking in membrane immunoglobulin-mediated, major-histocompatibilityrestricted T cell-B cell cooperation. J. exp. Med., 162, 1695. WHA~.EN,B., ToNy, H.-P., & PARKER,D.C. (1988), Characterization of the effector mechanism of help for antigen-presenting and bystander resting B-cell growth mediated by la-restricted Th2 helper T-cell lines. J. Immunol., 141, 2230.
Res. lmmunol. IBt% 19:,,.,, 141,405-454
Paris 1990
32nd FORUM IN IMMUNOLOGY
H O W D O E S T H E H E L P E R T C E L L ACTIVATE T H E R E S T I N G B C E L L W H E N liT R E C O G N I Z E S A N T I G E N ON T H E B - C E L L S U R F A C E ?
Organized by D.C. Parker
INTRODUCTION
The resting B lymphocyte is all potential, a container for a unique set of antibody genes created by DNA recombination in the bone marrow. To express its potential as secreted antibody, the B cell must be activated to proliferate and differentiate into a clone of antibody-secreting cells. Activation depends upon binding of antigen to membrane immunoglobulin (lg) on the B cell. The result is a specific antibody response by the process of clonal selection, since the membrane Ig is a sample of the antibody that the clone will secrete. This simple picture is complicated by the fact that a normal antibody response to a protein antig, en requires the participation of helper T lymphocytes specific for the same antigen particle. The question of how T cells help B cells has kept a number of immunologists occupied for two decades. Con-
siderable progress has bee.~ mad.e with this problem, which reveals new leve!s of complexity as it unfolds. The new consensus holds that B cells get help by acting as antigen-specific antigen-presenting cells, an idea that was proposed in the mid-seventies as soon as the outlines of how MHC antigens guide T-cell recognition on cell surfaces became apparent (Sprent, 1978; Benacerraf, 1978), but was not directly demonstrated until nearly a decade later (Tony and Parker, 1985; reviewed in Chestnut and Grey, 1986). This requirement for direct cellular interaction between an antigen-presenting B cell and a helper T cell explains MHC-restriction of T-cell help. Since B lymphocytes can recognize native proteins and then process them for presentation, it also explains how B cells can make anti
406
32 nd F O R U M I N I M M U N O L O G Y
bodies that show exquisite specificity for protein conformation while receiving help from T cells that recognize small peptides. What remains to be resolved is the mechanism of help" which molecules and activation pathways does the helper T cell use to drive the B cell into proliferation and Ig secretion? That is the question addressed by the scientists who have contributed to this Forum. As noted in the comments by some of the centributors, this Forum produced considerable agreement but not consensus on a number of important issues. !) The role of membrane Ig, the antigen receptor of the B cell, is largely to enable antigen-specific antigen presentation. A signal from membrane Ig is not require.d for a vigorous antibody response, since the helper T cell can do the whole job once antigen is displayed on the B-cell surface in the context of MHC molecules. This result, although anticipated by some (Augustin and Coutinho, 19~0), is somewhat surprising in view of the well-documented signals that can be transmitted to B cells by cross-linking membrane lg. In fact, these signals may be more relevant to thymus-independent, anti-polysaccharide antibody respoe,~es. A membrane Ig signal can enhance a suboptimal T/B ." .
.
i
.
.
.
i. -~.
.
.
l l l t g ; l iat~tll,J l l ,
.
.
!,., _
___
pg;I l l i i p ~
L _ _
uy
increasing
cla~s ii expression, and may play a role in thymus-dependent responses in vivo, but in our own in vitro experiments with Th2 helper T-cell lines, we found no influence of membrane lg signalling on either the efficiency of antigen presentation by small B cells or their response to T-cell help (Tony et al., 1985). 2) Lymphokines acting in combination play an essential role, but the identified lymphokines by themselves are inadequate to induce clonal expansion and secretion by resting B cells. Most of us believe that some sort of cell contact is also required (although Julius points out that IL-5 appears to induce immunoglobulin secretion without proliferation in at least a proportion of small B cells, and Theze and Diu are actively pursuing a soluble factor that induces proliferation and secretion of resting B cells in their hands). In some
experiments, cell contact alone can induce at least the first round of DNA synthesis, while in other experiments, contact plus IL-4 or IL-2 is required. 3) Even in responses that are MHCrestricted at the T cell/B cell level, MHC restriction and antigen specificity have more to do with induction of helper activity by the B cell acting as an antigenpresenting cell than with the delivery of help to the B cell. This conclusion is based on contact-dependent activation of bystander small B cells (Whalen et al., 1988) as well as B-cell responses induced by T-cell lines and normal T cells activated by antibodies to the T-cell receptor complex in the absence of antigen or appropriate MHC alleles, as described in this Forum by Lipsky, Marshall and Noelle, Owens, Kehry and colleagues, Snow and colleagues, and Crow and Friedman. Swain and Croft point out that these responses are efficient only when the T cells are strongly activated, and so may represent only a minimal model for the delivery of help. Nevertheless, this result implies that the membrane molecules that provide the contact signals for help are distinct in part from those that activate the T ~ e i"l . In particular, if class I1 MHC molecules are involved in contact signal transduction, they would not need to be engaged ~,~r,|¢;,-,,,~ll,,
~ i ~ l
11~11~
h,, O~
~h,~ til~
T
~11
I "~ll
~ ; ~
~lltl~ii
receptor. 4) S~me component of the contact help machinery is inducible on the time scale of lymphokine synthesis and secretion, implying that help depends on induced synthesis or protein synthesisdependent modification of a membrane molecule on the helper T cell. The strongest statements on this theme are made by Noelle and colleagues, using activated, fixed helper T cells and by Kehry and colleagues using whole membranes of activated helper T cell~. Identification of this molecule would seem to be a top priority. By analogy with T-cell activation, it seems likely that multiple molecules will be involved in B-cell activation, including L F A - I / I C A M - I , CD2/LFA-3, CD45, and possibly CD4/class II. Other candidate molecules could be proposed. Antibody inhibition experiments are likely to be
HOW
T CELLS HELP B CELLS
informative, particularly if done with Fab fragments to minimize negative signalling through membrane molecules, but have not yet produced consistent results among the authors in this Forum. Looking further ahead, we can see that an adequate understanding of T-dependent B lymphocyte activation will require a description of how multiple external signals result in an integrated cellular response. The papers by Crow and Friedman, Snow and colleagues, and our own unpublished results suggest that contact help probably involves second messenger systems that are different from those induced by membrane Ig crosslinking. Unravelling causal connections among external signals, second messenger systems, and the cellular response will be difficult, because the biochemical changes occur in minutes, while T-cell help is a protract-
407
ed process, requiring up to 24 hours before most of the B cells are committed to a round of DNA synthesis, even in the presence of exogenous lymphokines. For optimal proliferation and differentiation to antibody secretion, T-cell help may need to be continuous over days (Owens). Therefore, as anticipated in some of the experiments described in this Forum, the B-cell response will need to be studied in stages (early events, stages of the cell cycle, and differentiadon ~o Ig secretion), and the requirements for passage from one stage to the ne~t studied individually. D.C. Parker Department of Molecular Genetics and Microbiology, Utfiversity of Massachusetts Medical School, Worcester, MA 01655 (USA)
References. At~t~t~sx~N,A.A. & Coux~Nuo, A. (1980), Specific T helper cells that activate B cells polyclonally. in vitro enrichment and cooperative function. J. exp. Med., 151,587. BENACERRAF,B. (1978), A hypothesis to relate the specificity of T lymphocytes and the activity of I region-specific IR genes in macrophages and B lymphocytes. J. Immunol., i20, i809. CHESNUT,R.W. & H.M. GREY(1986), Antigen presentation by B cells and its significance in T-B interactions. 4dvanc. Immunol., 39, 51. SeReNe, J. (1978), Restricted helper function of F~ hybrid T cells positively selected to heterologous erythrocytes in irradiated parental strain mice. - - II. Evidence for restrictions affecting helper cell induction and B-cell collaboration, both mapping to the K end of the H-2 complex. J. exp. Med., 147, 1159. TONY, H.-P. & PARKER,D.C. (1985), Major histocompatibility .co.mplex-restricted, polyclonal B-cell responses resulting from helper T-cell recognmon of anti-immunoglobulin presented by small B lymphocytes. J. exp. Med., 161, 223. ToNy, H.-P., PHIl.tiPS, N.E. & PARKE~, D.C. (1985), Role of membrane immunoglobulin crosslinking in membrane immunoglobulin-mediated, major-histocompatibilityrestricted T cell-B cell cooperation. J. exp. Med., 162, 1695. WHA~.EN,B., ToNy, H.-P., & PARKER,D.C. (1988), Characterization of the effector mechanism of help for antigen-presenting and bystander resting B-cell growth mediated by la-restricted Th2 helper T-cell lines. J. Immunol., 141, 2230.