T helper cells required for the in vitro primary antibody response to SRBC are neither SRBC-specific nor MHC-restricted.

These studies address the specificity of the T cells which normally function in the in vitro primary immune response. It is generally accepted that SRBC-specific, MHC-restricted T helper cells become activated both in vivo and in vitro by seeing erythrocyte antigens presented in the context of the MHC antigens of an antigen-presenting cell. At this point, the in vivo and in vitro systems differ. In vivo, the interaction between SRBC-specific B cells and the activated T helper cells is itself also MHC-restricted, whereas in vitro, the T cell-B cell interaction is not MHC-restricted, and may be factor-mediated. It is the first phase of the in vitro response, and actually the specificity of the T cells which function in the in vitro primary response that we are questioning, in these studies. The experimental approach was to deplete antigen-specific T helper cell activity by passage of T cells through irradiated mice in the presence of a high dose of antigen, and collecting thoracic duct cells one day later. Using this protocol, we have confirmed that removal of SRBC-specific T cells ablates the adoptive transfer in vivo response to SRBC. However, the same negatively selected T cell populations were just as potent as control T cells in supporting the in vitro response to SRBC. We confirmed that the T cells removed respond to SRBC in an MHC-restricted manner by removing from an F1 T cell population the cells able to respond to SRBC in association with one parental haplotype. These T cells nonetheless provided equal help for both parental B cell populations in vitro. These experiments show that the MHC-restricted antigen-specific T cell which is required for the in vivo response is not required in the primary in vitro response to erythrocytes. The significance of this is twofold. Firstly, this demonstrates clearly that different T cells are functioning in vitro vs. in vivo. Singer and his colleagues have shown that different B cell subpopulations (Lyb5+ vs. Lyb5-), with differing MHC-restriction requirements, are preferentially activated in vivo and in vitro. Our data says that another difference between the in vivo and in vitro SRBC response is that different T cells are working in the two systems. Understanding exactly which B cell and T cell subpopulations are working within the model system (in vivo or in vitro) that an investigator chooses is obviously of critical importance.(ABSTRACT TRUNCATED AT 400 WORDS)