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Incomplete Activation of CD4 T Cells by Antigen-Presenting Transitional Immature B Cells: Implications for Peripheral B and T Cell Responsiveness¹

James B. Chung^*, Andrew D. Wells, Scott Adler, Anand Jacob, Laurence A. Turka and John G. Monroe^2,

Divisions of ^* Rheumatology and Nephrology, Department of Medicine, and Department of Pathology and Laboratory Medicine, University of Pennsylvania, Philadelphia, PA 19104

B cells leave the bone marrow as transitional B cells. Transitional B cells represent a target of negative selection and peripheral tolerance, both of which are abrogated in vitro by mediators of T cell help. In vitro, transitional and mature B cells differ in their responses to B cell receptor ligation. Whereas mature B cells up-regulate the T cell costimulatory molecule CD86 (B7.2) and are activated, transitional B cells do not and undergo apoptosis. The ability of transitional B cells to process and present Ag to CD4 T cells and to elicit protective signals in the absence of CD86 up-regulation was investigated. We report that transitional B cells can process and present Ag as peptide:MHC class II complexes. However, their ability to activate T cells and elicit help signals from CD4-expressing Th cells was compromised compared with mature B cells, unless exogenous T cell costimulation was provided. A stringent requirement for CD28 costimulation was not evident in interactions between transitional B cells and preactivated CD4-expressing T cells, indicating that T cells involved in vivo in an ongoing immune response might rescue Ag-specific transitional B cells from negative selection. These data suggest that during an immune response, immature B cells may be able to sustain the responses of preactivated CD4⁺ T cells, while being unable to initiate activation of naive T cells. Furthermore, the ability of preactivated, but not naive T cells to provide survival signals to B cell receptor-engaged transitional immature B cells argues that these B cells may be directed toward activation rather than negative selection when encountering Ag in the context of a pre-existing immune response.

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