Molecular composition of an antigen-specific, Ly-1 T suppressor inducer factor. One molecule binds antigen and is I-J-; another is I-J+, does not bind antigen, and imparts an Igh-variable region-linked restriction.

Immunized Ly-1+2-T cells (Ly-1 cells) make an antigen-specific soluble suppressor product (Lyl-1 TsiF) that will induce Ly-2+ cells to express suppressive activity but only if the Ly-2+ and the Ly-1 producer cell share genetic polymorphisms that are linked to the Igh locus and in particular that part where the Igh-V (or VH) is encoded. Ly-1 TsiF can be separated into entities, one binds antigen and does not express I-J determinants, and the other is I-J+ and does not bind antigen. Neither of these "subfactors" has biological activity, but a 50:50 mixture of them reconstitutes biological activity that expresses the antigen specificity of the antigen-binding molecule. Any of the three heterologous erythrocytes (antigens) studied can be used for immunization to produce the I-J+ nonantigen-binding factor, i.e., the I-J+ moiety makes no contribution to the factor's specificity. It does, however, determine the intact factor's Igh-V linked restriction. Thus, the antigen combining site of the factor is irrelevant to the factor's Igh-V restriction but crucial for its specificity. The I-J+ molecule does not bind antigen nor influence the factor's antigen specificity but expresses the Igh-V polymorphism (or anti-Igh-V polymorphism) that is required for the transmission of an inductive signal to the factor's Ly-2+ acceptor cell.