Characterization and activity of Fas ligand producing CD5⁺ B cells

B lymphocytes make several contributions to immune regulation including production of antibodies with regulatory properties, release of immune suppressive cytokines, and expression of death-inducing ligands. A role for Fas ligand (FasL)-expressing "killer" B cells in regulating T helper cell survival and chronic inflammation has been demonstrated in animal models of schistosome worm infection, asthma, and autoimmune arthritis. Interestingly, a population of CD5(+) B cells found in the spleen and lungs of naïve mice constitutively expresses FasL and has potent killer function against T helper cells that is antigen-specific and FasL-dependent. Killer B cells therefore represent a novel target for immune modulation in many disease settings. Our laboratory has recently published methods of characterizing FasL(+) B cells and inducing their proliferation in vitro. This chapter will describe detailed methods of identifying and expanding killer B cells from mice, detecting FasL expression in B cells, and performing functional killing assays against antigen-specific TH cells.