Cutting edge:: B cells promote CD8+ T cell activation in MRL-Faslpr mice independently of MHC class I antigen presentation

Spontaneous CD8(+) T cell activation in MRL-Fas(lpr) mice is B cell dependent. It is unclear whether this B dependent activation is mediated by direct Ag presentation via MHC class I proteins (i.e., cross-presentation) or whether activation occurs by an indirect mechanism, e.g., via effects on CD4(+) cells. To determine how CD8(+) T cell activation is promoted by B cells, we created mixed bone marrow chimeras where direct MHC class I Ag presentation by B cells was abrogated while other leukocyte compartments could express MHC class I. Surprisingly, despite the absence of B cell class I-restricted Ag presentation, CD8+ T cell activation was intact in the chimeric mice. Therefore, the spontaneous B cell-dependent CD8(+) T cell activation that occurs in systemic autoimmunity is not due to direct presentation by B cells to CD8(+) T cells.