A series of 27 B-cell lymphomas (designated the CH series), induced in B1O.H-2aH4bp/Wts mice by intense adoptive immunization with sheep erythrocytes, was found to represent a subset of the total B-cell repertoire. This subset was characterized by expression of a limited number of Ig heavy chain variable regions, as evidenced by the presence of crossreactive idiotypes and common antigen binding specificities. Twenty-one of the 27 CH lymphomas studied were classified into five groups, defmed by a particular cross-reactive idiotype; four of these groups were linked in a single network. Seven of 16 idiotypes defined by absorption analysis were present on lymphomas bearing either K or A light chains and so were localized to the heavy chain variable region. The surface Ig on 14 CH lymphomas was found to be specific for epitopes on certain erythrocytes (bromelain-treated autologous erythrocytes, sheep, and chicken erythrocytes) or E. coUl. We propose that the CH lymphomas represent the malignant counterparts of a subset of idiotypically related, normal B cells in B1O.H2aH4bp/Wts mice. Perturbation of this idiotype network, by hyperimmunization with an antigen for which some of the members are specific (sheep erythrocytes), increases the risk for neoplasia. Possible mechanisms for this are discussed.

The role of cell surface immunoglobulin in helper T-cell-dependent B-cell activation was analyzed using a B-cell lymphoma, CH12, with known antigen specificity and activation properties similar to those of a resting B cell. Two sources of helper T cells were used, both selected such that they interact with H-2-encoded determinants on CH12 in the absence of the specific B-cell antigen, sheep erythrocytes. By this dissociation of the specificity of the T cells from that of the B cells, the requirement for antigen in the induction of CH12 to antibody secretion could be studied. The results show that both helper T-cell-B-cell interactions and surface immunoglobulin-antigen binding are involved in inducing B-cell differentiation, thus establishing a signalling function for the antigen receptor on B lymphocytes. Our data also show that the requirement for surface immunoglobulin-ligand interactions in B-cell activation can, under certain conditions, be circumvented, notably when high (nonphysiologic) multiplicities of Tcell help are used.The binding of antigen to surface immunoglobulin (sIg) on the B-cell membrane facilitates the interaction of antigenspecific helper T (Th) cells with specific B cells by "bridging" the two cell types. Whether the function of the sIg molecule is to passively focus T-cell help onto the B-cell membrane (1) or whether the binding of antigen to sIg also fulfills a signalling function (2) has been a matter of contention. Resolution of this controversy requires that the Th-cell and Bcell specificities for antigen be dissociated, and T-cell specificity must be selected such that efficient T-B interaction occurs independently of the involvement of sIg. Such a method is suggested by the work of Augustin and Coutinho (3) MATERIALS AND METHODS Animals. B10.H-2aH-4bp/Wts (2a4b) mice were bred and maintained under pathogen-free conditions at the University of North Carolina. (BALB/c x C57BL/6)F1 (CB6F1, H2d/b), BALB/c (C, H-2d), and C3H/HeJ (C3H, H-2) mice were purchased from The Jackson Laboratory and maintained in the animal colony at Duke University. All mice were 8-to 12-wk old when used. CH12 Lymphoma Cells. CH12 arose in a 2a4b mouse and was propagated as an ascites tumor. The induction, characterization, and identification of the tumor cells' specificity for SRBC have been described (10). From the ascites, 94.3 + 5.5% of the cells were ,u by immunofluorescence (10 experiments) while 1.2 ± 0.4% were nonspecific esterase positive (four determinations). The role, if any, that these latter cells in the tumor preparation might play in the activation of CH12 has not been analyzed. In 8 experiments, 89.9 ± 6.4% of the cells rosetted with SRBC but none with human erythrocytes or mouse erythrocytes (MRBC). We have recently found that this apparent specificity for SRBC is likely to be a cross-reaction with autologous effete erythrocytes, because CH12 rosettes as efficiently with bromelain-treated MRBC (12) as with SRBC. Greater than 95% of cells are positive for the CH12 idiotype, as determined...

The influence of beta-chain diversity on the expressed T-cell receptor (TCR) alpha-chain repertoire was investigated using transgenic mice which exclusively express a single rearranged TCR beta-chain gene. Analysis of these mice using alpha-chain-specific recombinant cDNA libraries showed that expression of the transgene-encoded beta chain results in significant skewing in Tcra-V gene segment usage vs nontransgenic mice. Skewing was most pronounced towards alpha chains using TCRA-V segments. Sequence analysis of Tcra-V8-containing genes from transgenic T cells revealed predominant use of a single Tcra-J segment (Tcra-J24), which was not detected in Tcra-V8 containing genes isolated from nontransgenic T cells. Further analysis revealed that co-expression of Tcra-V8 with Tcra-J24 in beta-transgenic mice is exhibited almost exclusively by CD4+ T cells, and is associated with a limited number of closely related N-regions. Analysis of transgenic CD8+ T cells demonstrated predominant co-expression of Tcra-V8 with another Tcra-J (Tcra-J30), together with a different, limited N-region sequence. We conclude that the composition of expressed beta chains can profoundly influence the selection of companion alpha chains expressed in the periphery, and that alpha-chain N and J regions play a crucial role in discriminating between class I vs class II major histocompatibility complex (MHC)-restricted recognition. Further, these results are in agreement with recent data concerning the crystal structure of the TCR, and most consistent with a model for TCR structure in which the complementarity determining region (CDR)3alpha domain participates in direct contact with the MHC.

We have evaluated the role of major histocompatibility complex-encoded class II (Ia) molecules as transmembrane signaling receptors in the T helper cell-dependent activation of B lymphocytes. For these studies, we utilized the murine B-cell lymphoma CH12, which expresses both I-A and I-E class II molecules. In addition, CH12 cells carry IgM of known antigen specificity and require both specific antigen and Ia-restricted T-cell help for the induction of antibody secretion. In this respect, they resemble normal resting B cells. We have studied the ability of antigen-specific or alloreactive T helper cells reactive with either the I-A or the I-E molecules on CH12 to be activated and their ability to stimulate antibody production by CH12. The results show that, although CH12 cells present antigen to T helper cells that interact with either the I-A or the I-E molecules, CH12 cells are stimulated to secrete antibody only by T helper cells reactive with their I-E molecules. Our data demonstrate that class II molecules are transducers of signals for B-cell excitation in addition to serving a restricting function for helper T-cell stimulation. Moreover, the data demonstrate that these two functions, T-cell stimulation and B-cell excitation, are discrete and need not be expressed by the same Ia molecule.

The question of whether surface immunoglobulin and Ia molecules have a signalling function in helper T cell-dependent activation of B cells has been evaluated. Two sources of B cells have been used, one a purified population of hapten-binding B cells, the other a B-cell lymphoma, CH12, with known antigen specificity. Evidence is presented that both immunoglobulin and Ia molecules are receptors actively involved in the initial activation of resting B cells. Nevertheless, the requirements for ligand binding to either receptor can be bypassed under appropriate conditions, and the implications of this result for the function of these molecules is discussed. With respect to B-cell Ia, the authors present data that demonstrate two distinct functions of this molecule, one as a restricting element for T-cell activation, the second as a signalling receptor for B-cell excitation. On the CH12 surface, the I-A molecule fulfills the former function, but T-cell interactions with I-A fail to result in B-cell stimulation, suggesting that B-cell Ia may limit helper T cell-B cell interactions. We suggest that the binding of antigen surface immunoglobulin and binding of helper T-cell receptors to the appropriate Ia molecule(s) results in the activation of genes that encode for a third class of membrane B-cell receptors, those that bind B-cell stimulating factors.