Fig. 4.

Naïve spleen, but not naïve PerC, produce anti-FtL primary responses to FtL challenge when transferred intravenously to naïve recipients. (Top) 10⁷ PerC (from one to two naïve mice) or indicated number of spleen cells from naïve Igh^a mice were transferred intravenously to naïve congenic Igh^b recipients immunized with FtL the next day. Donor and recipient anti-FtL responses were measured 5 d later. (Middle) Live B cells from indicated recipient spleen were gated to identify donor anti-FtL B-1a (IgM^a+) (circled population, Top), which are then further gated to show donor anti-FtL plasma cells (CD267^hiCD138⁺, boxed population, Rightmost Middle). CD267, transmembrane activator, and calcium signal-modulating cyclophilin ligand interactor (TACI) is a marker induced during plasma cell differentiation. The boundary for FtL binding is determined from the fluorescence-minus-one (FMO) (40) staining control in which fluorochrome-labeled FtL was omitted from the stain-set (Rightmost Top). Live PerC anti-FtL B-1a were gated to distinguish donor or recipient-derived cells (boxed populations, Middle). (Bottom) Numbers of donor (IgM^a) anti-FtL B-1a in recipient spleen (Left); donor-derived (IgM^a) anti-FtL in sera of recipients (Right). Each point represents data from an individual mouse (n = 5, per group). Values are expressed as microliter equivalents of a standard serum pool from 5-d FtL-primed Igha mice.