Interactions between immunoglobulins and I region antigens on the B lymphocyte membrane

Immunoglobulin receptor signaling depends on the carboxyl terminus but not the heavy-chain class

To examine the isotypic and structural requirements involved in signaling through the immunoglobulin (Ig) receptor on B lymphocytes, we established a panel of T15 idiotype-positive transfectants that expressed wild-type IgM, wild-type IgD, or hybrid IgM molecules. Growth inhibition of the transfected lymphoma cells in response to anti-idiotype antibodies was used to measure signaling. Hybrid IgM molecules were constructed so that the membrane-spanning region of the mu heavy chain was replaced by that of delta, gamma 2b, or alpha heavy chains or that of the I-Ab class II (Ia) alpha chain. All transfectants that expressed IgM or hybrid IgM molecules with membrane-spanning regions from another Ig isotype underwent signaling in response to anti-idiotype antibodies, whereas the IgM-Ia hybrid transfectants did not. Transfectants that expressed wild-type IgD molecules also underwent signaling, although this response was particularly sensitive to serum concentrations. These results imply that signaling occurs in a similar manner through heavy-chain receptors of any isotype and suggest that conserved amino acid sequences in the transmembrane regions are important in this process.

A model system for the analysis of B-cell activation and effector T-cell functions. T cell-dependent B-cell responses facilitated by anti-I-A antibodies

We have attempted to develop an in vitro system where polyclonal B lymphocyte responses could be induced in 'antigen-like' conditions, that is, where surface immunoglobulin-dependent binding mediates interaction with a mitogen. Monoclonal anti-mu and anti-delta antibodies were covalently bound to lipopolysaccharide (LPS) and these complexes were shown to display mitogenic activity. Polyclonal plaque-forming cell (PFC) responses, however, were diminished in cultures stimulated by anti-mu-LPS (but not by anti-delta-LPS) indicating that 'anti-mu inhibition' of terminal B-cell differentiation also applies to 'specific' antibody responses. Moreover, the analysis of the functional activity of monoclonal antibodies to major histocompatibility complex (MHC) class II molecules revealed a surprising synergy between low, non-stimulatory concentrations of anti-mu-LPS (but not anti-delta-LPS) with anti-I-A antibodies. These responses are T-cell dependent and synergy with anti-mu-LPS conjugates can also be obtained with 'naturally' activated CD4+ cells isolated from normal donors. A model of molecular and cellular interactions was derived, which accounts for the present findings and is applicable in antigen-dependent lymphocyte collaboration.

Maturation of the immune response: a computational model

Experimental studies of the effect on antibody affinity of antigen dose and time after immunization show that average affinity increases progressively with time after immunization, and that this increase is greater at lower doses of antigen. In this paper we describe a polyclonal computer model of the immune system that yields all the essential phenomena of affinity maturation, including dose-dependency. Our main findings are (1) the dose-dependency relationship is not produced when typical assumptions regarding B-cell populations and binding reactions are employed, and (2) it is possible to reproduce this dependency by assuming two classes of lymphocytes: generalists and specialists. Generalists have a low threshold for response and produce antibody of low effectiveness, whereas specialists have a high threshold for response, and produce highly effective antibody. We make an analogy between the generalists and a pioneer species in ecological succession, and suggest how the generalists may contribute to a more effective defense against real infections.

Role of HLA class I and class II antigens in activation and differentiation of B cells

The monoclonal antibodies (MoAb) CR10-214, CR11-115, and Q1/28 to distinct monomorphic determinants of HLA class I antigens, the MoAb CL413 and PTF29.12 recognizing monomorphic determinants of HLA-DR antigens, the anti-HLA-DQw1 MoAb KS11, the anti-HLA-DPw1 MoAb B7/21, and the anti-HLA-DR,DP MoAb CR11-462 were tested for their ability to modulate human B-lymphocyte proliferation and maturation to IgM-forming cells. Purified tonsillar B cells were stimulated with Staphylococcus aureus bacteria of the Cowan first strain (SAC) or anti-human mu-chain xenoantibodies, as well as in growth factor- or T-cell-dependent activation cultures. The B-cell proliferative responses induced by SAC or by mitogenic concentrations of anti-mu-chain xenoantibodies were inhibited by some of the anti-HLA class I and anti-HLA class II monoclonal antibodies tested. The same antibodies were effective inhibitors of the proliferation of B cells stimulated with interferon-gamma (IFN-gamma) or interleukin-2 (IL-2) and with submitogenic concentrations of anti-mu-chain xenoantibodies. The proliferation induced by IL-2 of SAC-preactivated B cells was inhibited by some of the anti-HLA class II monoclonal antibodies, but not by the anti-HLA class I monoclonal antibodies tested. This inhibition appeared to reflect at least in part a direct effect on later events of the B-cell activation cascade, since some anti-HLA class II monoclonal antibodies still exerted considerable inhibitory activity when added together with IL-2 to SAC-preactivated B cells after the third day of culture. Anti HLA-DR, DQ, and DP monoclonal antibodies consistently inhibited the IgM production induced in B cells by T cells alone, T cells plus pokeweed mitogen (PWM), SAC plus IL-2, or IL-2 alone. In contrast, two of the three anti-HLA class I monoclonal antibodies tested inhibited the IgM production in cultures stimulated with SAC plus IL-2 and one the IgM production induced by IL-2 alone, but none of them had inhibitory effects on T-cell dependent IgM production. The results reported herein indicate that HLA class II molecules directly participate in different phases of the B-cell activation cascade. In addition, our data also suggest that HLA class I molecules can be involved in the events leading to B-cell proliferation and differentiation into immunoglobulin-secreting cells.

The role of I-A/E molecules in B-lymphocyte activation. II. Mechanism of inhibition of the responses to lipopolysaccharide by anti-I-A/E antibodies

We have previously demonstrated that monoclonal anti-I-A/E antibodies inhibit B-cell responses to lipopolysaccharide (LPS). In the present report, the inhibitory effects were shown to be carried out directly on B cells, and to be totally independent of the LPS concentration used, thereby showing that antibodies do not mediate their effect through blocking of accessory cells or steric hindrance of LPS-receptors. Of the three different phases in B-cell activation/induction, proliferation, and maturation, induction was shown to be the most sensitive to inhibition by anti-I-A/E antibodies. Thus, kinetic studies showed that anti-I-A/E antibodies are only inhibitory for the first 16 h of LPS activation, after which B cells can no longer be inhibited by these antibodies. Class II MHC molecules appear, therefore, to be part of a membrane molecular complex which regulates delivery of activation signals to resting B cells. Since it was also shown that this time period corresponds approximately to the time required for B cells to express functional reactivity to growth factors, we suggest that anti-I-A/E antibodies act on resting B lymphocytes to inhibit mitogen-dependent induction of growth receptor expression.

The role of immunoglobulin receptors in "cognate" T-B cell collaboration

The functional effects of anti-Ig antibodies have been investigated, using an experimental system where B cell activation is brought about by direct and specific interactions with T helper (Th) cells without participation of surface Ig receptors on the responding B cell. We have used Th cell lines and clones directed to class II major histocompatibility complex antigens of the responding B cells, and titrated into cooperative cultures either purified rabbit anti-mouse mu, or monoclonal mouse anti-delta antibodies. Both types of antibodies greatly enhanced B lymphocyte responses to suboptimal concentrations of functionally efficient Th cells, while they had no effect in cultures containing optimal Th:B cell ratios. In contrast, helper activity by low efficiency Th was, at all Th:B cell ratios, enhanced by appropriate concentrations of anti-Ig antibodies. Anti-Ig effects were exclusively observed when B cells were the targets for "cognate" recognition by Th cells. We conclude that ligand binding to surface Ig receptors on resting B cells fails, in our experimental conditions, to overcome "linked" collaboration, but it greatly facilitates productive Th-B cell interactions. Whatever the mechanisms underlying this facilitation, the observations imply roles of surface Ig in Th-dependent B lymphocyte activation other than either passive "focusing" of antigen or activation into reactivity to soluble, unspecific factors.

Anti-immunoglobulin stimulation of human B lymphocytes is inhibited by anti-class II major histocompatibility complex antibodies

The effect of murine monoclonal antibodies binding monomorphic epitopes of Class II, HLA-DR molecules on responding human B lymphocytes stimulated by anti-immunoglobulin M (IgM) antibodies was studied. Goat F(ab')2 anti-human IgM coupled to Sepharose beads (insoluble), or in solution, was added to macrophage-depleted B cells in culture with, or without, anti-human HLA-DR monoclonal antibodies. The addition of monoclonal anti-HLA-DR antibodies to anti-human IgM-stimulated B lymphocytes inhibited this T-independent B-cell proliferation by 82-94%. The role of Class II, HLA-DR molecules on B cells may therefore exceed that of antigen presentation alone, to include responding B-cell activation induced by anti-immunoglobulin.

Interactions of small B lymphocytes with unprimed noncytolytic T cells: dissociation between "presentation" and growth induction

The accessory cell requirements in lectin-dependent triggering and growth of unprimed Lyt-2-T lymphocytes were analyzed by quantitatively comparing the ability of small B cells and peritoneal macrophages to either induce reactivity to growth factors or support growth. Lightly or nonirradiated small B cells were 15 to 30-fold less efficient as compared to T cell-depleted peritoneal cell populations, in the support of the lectin-stimulated Lyt-2-T cell proliferation. In contrast, lightly irradiated small B lymphocytes were quantitatively as efficient as macrophages in mediating lectin-driven Lyt-2-T cell proliferation, if relevant supernatants were added into culture. Finally, supernatants derived from cultures where T-small B cell ratios were optimal for growth of responder Lyt-2-lymphocytes were two orders of magnitude less efficient than conditioned medium obtained from cultures containing optimal T-macrophage ratios, in their ability to support growth of activated T cells. We conclude from these experiments that: in contrast to cytolytic T cell precursors, lectin-dependent induction of unprimed Lyt-2- T lymphocytes requires accessory cells; small B cells and macrophages are equally competent in this respect; and growth support by small B cell populations is due to contamination by macrophages which are the only cell type performing this function. We therefore interpret reports on Lyt-2- T cell proliferation upon stimulation with high numbers of small B cells as a two-step process: "presentation" and induction of T cells which is essentially B cell dependent, and factor production ensured by contaminating macrophages.

B-cell growth factor (B-cell growth factor I or B-cell-stimulating factor, provisional 1) is a differentiation factor for resting B cells and may not induce cell growth

B-cell growth factor I [BCGF I or B-cell-stimulating factor, provisional 1 (BSFp1)] has been defined as a T-cell-derived lymphokine that acts as a co-stimulator of polyclonal B-cell growth in B cells cultured with anti-mu, anti-delta, or anti-Ig. Based on a number of studies it has been suggested that anti-Ig induces cell enlargement, entry into the G1 phase of the cell cycle, and expression of receptors for BSFp1. BSFp1 then induces entry of the cells into S phase. By adding BSFp1 prior to anti-Ig, we have found evidence that BSFp1 renders cells susceptible to anti-Ig-mediated entry of cells into G2/S phase. In contrast, if cells are first treated with anti-Ig, washed, and then cultured with BSFp1, they do not enter S phase. Taken together, these results suggest that BSFp1 acts on the resting B cells not as a growth factor but rather as a lymphokine that prepares cells for anti-Ig-mediated activation. Taken together with previous reports that BSFp1 induces increased expression of Ia antigens on resting B cells, these studies suggest that BSFp1 may be a differentiation factor rather than a growth factor and that it acts on resting B cells.