*Purpose: Therapies involving CD4 regulatory T cells (Tregs) are promising approaches to mitigate graft rejection and autoimmunity. Understanding of the activation mechanisms that initiate Treg development in specific thymic niches and potentiates suppression in the periphery will facilitate Treg application. Contrary to other thymic antigen-presenting cells, thymic B cells were recently found to cluster around Treg precursors in the thymic medulla. As compared to splenic B cells, thymic B cells were shown to better activate Tregs. We hypothesized that thymic B cells, clustered in dedicated medullary niches, present peptides on MHC class II molecules (MHC-II) to maturing Treg precursors.

*Methods: To test the hypothesis we developed a new cell-targeted gene knock-out mouse, in which MHC-II expression was blocked only in B cells. We derived the MHC-II IAα ^fl/flmutant mouse on the C57BL/6 background (IAαβ^b, IE⁰). This mouse was bred with and backcrossed to the CD19-Cre^+/+ mouse to obtain the Cl2-Δ-B mouse (IAα ^fl/fl; CD19-Cre ^+/-). IAα gene disruption was monitored by DNA-PCR and MHC-II IA^bexpression confirmed by flowcytometry (FCM). Phenotypic analysis measured the frequency and numbers of effector and regulatory CD4 T cells as well as B cells isolated from blood, thymus, and spleen. Lymphocyte activation was evaluated by FCM for expression of MHC-II, CD69, CD44 and CD86. Phospho-FCM was also performed to document lymphocyte activation via kinase signaling (pS6, pAKT and pERK). We are also developing a more stringent cell-targeted MHC-II KO model based on the mb1-Cre x IAα ^fl/fl combination.

*Results: The vast majority (93+/- 1.5 %) of B cells from young Cl2-Δ-B mice showed no MHC-II expression. Gene disruption was, however, not complete as reported in other models using CD19-Cre. Some CD19⁺ B cells precursors escaped Cre activity, expressed normal levels of MHC-II and reconstituted nearly half of the B cell compartment by 25 weeks. Surprisingly, 6-week-old Cl2-Δ-B mice showed a profound deficit in B cell numbers and frequencies but no decline in CD4⁺T cells. As this B cell drop was not seen in CD19-Cre-mediated deletion of other genes or in full MHC-II knock-out mice that have no CD4 T cells, this suggests that B cell development relies upon the sensing of pre-B cell MHC-II by CD4 T cells. Thymic MHC-II^negB cells also exhibited a resting phenotype (CD69^low, CD44^low, CD86^low), suggesting a deficit in antigen processing. Finally, Tregs from young Cl2-Δ-B mice showed a significant reduction in TCR fitness during activation

*Conclusions: In sum, we here provide evidence in support of a critical role of antigen-presentation by B cells in both B cell development and Treg maturation.

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