Impaired humoral immune responses in p110δ^D910A/D910A CD28^−/− mice. (A) WT, p110δ^D910A/D910A, CD28^−/−, CD28^Y170F, p110δ^D910A/D910A CD28^−/−, and p110δ^D910A/D910ACD28^Y170F mice were immunized with DNP-coupled KLH adsorbed onto alum. The mice were rechallenged with the same dose of antigen 78 days later. Anti-DNP–specific antibodies were detected by enzyme-linked immunoabsorbent assay. Each dot represents one mouse. (B) CD28 and p110δ are indispensable for efficient killing of allogeneically mismatched lymphocytes in vivo. Recipient mice on the B6 background (H-2^b) were injected with a mixture of 5 × 10⁶ CFSE^high B6 lymphocytes and CFSE^low CB6 F1 lymphocytes (H-2^b/k). The ratio of CFSE^low to CFSE^high was normalized to 1 on day 1, and the average from 3 recipients from each genotype is shown (± SD).