Effect of anti-delta antibodies on clonal expansion and maturation of B lymphocytes

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.

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.

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

Modulation and co-modulation of membrane-bound IgM and IgD and of I region-encoded antigens (I-A and I-E) were studied by immunofluorescence with the use of monoclonal antibodies. The two sets of molecules displayed different ability to be redistributed, by divalent ligands: immunoglobulins could be easily redistributed, while this was not the case for I-A and I-E molecules. After short periods of activation, however, this property was found to change drastically: I-region antigens were redistributed into polar caps, and immunoglobulins were modulated without clear polar redistribution. Cross-linking and removal of one of the molecules always resulted in the modulation of at least one other, often two membrane molecules. Concomitantly, an additional molecule was expressed in higher amount. The results are compatible with the existence of specific interactions at the level of the B cell membrane, involving the four molecules in the order I-A----IgM----I-E----IgD. Such interactions, probably reversible in the unperturbed state, are likely to change in affinity when one molecule interacts with a ligand. The possible relevance of membrane interactions between immunoglobulin and class II antigens in the regulation of the B cell is discussed.