B Cell-specific Deficiencies in mTOR Limit Humoral Immune Responses1

Generation of high-affinity antibodies in response to antigens/infectious agents is essential for developing long-lasting immune responses. B cell maturation and antibody responses to antigen stimulation require immunoglobulin (Ig) somatic hypermutation (SHM) and class-switch recombination (CSR) for high-affinity responses. Upon immunization with either the model antigen NP-CGG or heat-killed Pn14 derived from Streptococcus pneumoniae , knock-in (KI) mice hypomorphic for mTOR function had decreased ability to form germinal centers, develop high-affinity anti-NP or –Pn14 specific antibodies, and perform SHM/CSR. Hypomorphic mTOR mice also had a high mortality rate (40%) compared to WT (0%) littermates and had lower PspA specific antibody titers when immunized and challenged with live S. pneumoniae infection. Mice with mTOR deleted in their B cell lineage (KO) also produced fewer splenic germinal centers and decreased high-affinity antibody responses to NP-CGG than their WT littermates. CSR rates were lower in mTOR KI and KO mice, and pharmacologic inhibition of mTOR in WT B cells resulted in decreased rates of ex vivo CSR. RNA and protein levels of activation-induced cytidine deaminase (AID), a protein essential for SHM and CSR, were lower in B cells from both KI and B-cell specific KO mice, concomitant with increases in phosphorylated AKT and FOXO1. Rescue experiments increasing AID expression in KI B cells restored CSR levels to those in wild-type B cells. Thus, mTOR plays an important immunoregulatory role in the germinal center, at least partially through AID signaling, in generating high affinity antibodies.