Generation of high-affinity Abs in response to Ags/infectious agents is essential for developing long-lasting immune responses. B cell maturation and Ab responses to Ag stimulation require Ig somatic hypermutation (SHM) and class-switch recombination (CSR) for high-affinity responses. Upon immunization with either the model Ag 4-hydroxy-3-nitrophenylacetyl hapten (NP) conjugated to chicken γ globulin lysine (NP-CGG) or heat-killed Streptococcus pneumoniae capsular type 14 protein (Pn14), knock-in (KI) mice hypomorphic for mTOR function had a decreased ability to form germinal centers, develop high-affinity anti-NP–specific or anti-Pn14–specific Abs, and perform SHM/CSR. Hypomorphic mTOR mice also had a high mortality (40%) compared with wild-type (WT) (0%) littermates and had lower pneumococcal surface protein A–specific Ab titers when immunized and challenged with live S. pneumoniae infection. Mice with mTOR deleted in their B cell lineage (knockout [KO]) also produced fewer splenic germinal centers and decreased high-affinity Ab responses to NP-CGG than did 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 WT B cells. Thus, mTOR plays an important immunoregulatory role in the germinal center, at least partially through AID signaling, in generating high-affinity Abs.

Mammalian target of rapamycin (mTOR) regulates cell growth, cell cycle, and survival through two main complexes, mTORC1 and mTORC2. In our previous studies, striking decreases in both size and number of B cells were seen in mice with constitutive reductions in protein levels of both mTOR complexes. In addition, there were alterations in B cell differentiation with partial blocks in B cell development in the bone marrow. In our current studies, we have focused on their defect in antibody production. When challenged with either NP-CGG (T-dependent antigen) or NP-LPS (T-independent antigens), reduced IgG antibody titers to NP antigens were seen in mice with constitutive reductions in mTOR protein levels, despite equivalent levels of total antibody production; high affinity antibody production was also less. The numbers of germinal centers were reduced in the spleens of the mTORC1/2-compromised mice compared to their wild-type littermates. Fewer B cells from the mTORC1/2-compromised mice were able to undergo class switching from IgM to IgG. In addition, germinal center B cells from these mice exhibited reductions in somatic hypermutation rates of their immunoglobulin loci. Studies are currently underway to evaluate the ability of these mice to respond to an in vivo challenge of live bacteria. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 805. doi:10.1158/1538-7445.AM2011-805