A three-cell cluster hypothesis for noncognate T-B collaboration via direct T cell recognition of allogeneic dendritic cells.

In this article, we propose that T cell help for B cells can occur via an unusual three-cell cluster, with recipient CD4+ T helper cells interacting via direct allorecognition with donor dendritic cell class II MHC antigens, recipient B cells interacting with MHC class I (or any other) antigen on the donor dendritic cell surface, and noncognate (i.e., antigen nonspecific) T-B collaboration. In this noncognate pathway, antigen processing by B cells is not required and T cell help is potent because of the high precursor T cell frequency for direct recognition of allogeneic class II MHC molecules. The data supporting this hypothesis are: 1. LEW rat strain recipients of interstitial dendritic cell-free (DAxLEW)F1 kidney allografts were shown to have no detectable antibody to donor class I MHC antigens at day 7 after grafting. By contrast, LEW recipients of normal (DAxLEW)F1 kidneys had strong antibody responses. 2. Consistent wih important role for donor dendritic cells in the early antibody response to donor class I MHC antigens was the finding that it was dependent on donor class II MHC antigens. PVG recipients, previously immunized with pure DA RT1.B class II MHC antigens, had virtually no antibody response to the class I MHC antigens of DA kidney allografts. 3. We confirmed the low and high responder status of PVG and LEW rats, respectively, to DA class I antigens by studying antibody responses to pure DA class I antigens. However, PVG and LEW recipients of DA kidney allografts did not differ in their antibody response to the donor DA class I MHC antigens. This is consistent with this response not requiring the processing and presentation of DA class I antigen by PVG recipients. 4. LEW recipients of interstitial dendritic cell-free (DAxLEW)F1 kidney allografts did eventually develop a strong antibody response to DA class I antigens, but this was delayed by several weeks. That this delayed antibody response was probably mediated by conventional T-B collaboration and that T help was rate limiting in this situation, was demonstrated by immunizing LEW recipients with a DA class I peptide. This markedly accelerated the kinetics of the antibody response to the dendritic cell-free (DAxLEW)F1 kidneys.