Metabolic control of TFH cells and humoral immunity by phosphatidylethanolamine

T follicular helper (T_FH) cells are crucial for B cell-mediated humoral immunity¹. Although transcription factors such as BCL6 drive the differentiation of T_FH cells^2,3, it is unclear whether and how post-transcriptional and metabolic programs enforce T_FH cell programming. Here we show that the cytidine diphosphate (CDP)-ethanolamine pathway co-ordinates the expression and localization of CXCR5 with the responses of T_FH cells and humoral immunity. Using in vivo CRISPR-Cas9 screening and functional validation in mice, we identify ETNK1, PCYT2, and SELENOI-enzymes in the CDP-ethanolamine pathway for de novo synthesis of phosphatidylethanolamine (PE)-as selective post-transcriptional regulators of T_FH cell differentiation that act by promoting the surface expression and functional effects of CXCR5. T_FH cells exhibit unique lipid metabolic programs and PE is distributed to the outer layer of the plasma membrane, where it colocalizes with CXCR5. De novo synthesis of PE through the CDP-ethanolamine pathway co-ordinates these events to prevent the internalization and degradation of CXCR5. Genetic deletion of Pcyt2, but not of Pcyt1a (which mediates the CDP-choline pathway), in activated T cells impairs the differentiation of T_FH cells, and this is associated with reduced humoral immune responses. Surface levels of PE and CXCR5 expression on B cells also depend on Pcyt2. Our results reveal that phospholipid metabolism orchestrates post-transcriptional mechanisms for T_FH cell differentiation and humoral immunity, highlighting the metabolic control of context-dependent immune signalling and effector programs.

Immunological characterization of de novo and recall alloantibody suppression by CTLA4Ig in a mouse model of allosensitization

It is well known that CTLA4Ig inhibits allogenic T-cell activation in transplantation. The immunological features and mechanisms associated with alloantibody suppression by CTLA4Ig, however, are poorly understood. Here, we used a mouse model of allosensitization to evaluate the efficacy of CTLA4Ig (abatacept) in suppression of donor-specific antibody (DSA) during de novo and recall alloantibody responses. We found that abatacept inhibited de novo DSA IgM and IgG responses to HLA-A2 expressing skin grafts. Abatacept administered during primary T cell priming also reduced recall IgG responses induced by re-immunization. Suppression of de novo DSA responses by abatacept is associated with a reduction in splenic expression of the germinal center activation marker GL7 and a reduction of CD4(+)PD1(+)CXCR5(+) follicular T helper (Tfh) subset in splenic lymphocytes detected by flow cytometry. The efficacy of abatacept on recall DSA suppression is moderate. In vitro experiments demonstrated that abatacept inhibited DSA IgG secretion by CD138(+) plasma cells isolated from allograft recipients. Additional experiments using an IgG1 secreting mouse hybridoma cell line showed that abatacept binds to CD80 expressed on these cells with subsequent inhibition of cell proliferation and reduction in IgG ELISpot formation. In conclusion, CTLA4Ig is a potent suppressor of de novo DSA responses and also affects recall responses. The data suggests modification of recall DSA responses is due to a direct suppressive effect on plasma cells.

Monoclonal anti-interleukin-6 receptor antibody attenuates donor-specific antibody responses in a mouse model of allosensitization

Interleukin 6 is an immune regulatory cytokine that impacts the development and maturation of T-cell, B-cell, and antibody producing plasma cells. A monoclonal antibody to the IL-6R (Tocilizumab®) was recently approved by the FDA for treatment of rheumatoid arthritis. Although anti-IL-6R anitbodies can reduce autoantibody levels in human disease, the use of anti-IL-6R for alloantibody suppression has not been examined. Here, we report on our experience with a mousenized rat-anti-mouse IL-6R (mMR16-1) for attenuating donor-specific antibody (DSA) responses. C57BL/6 mice were sensitized with skin allografts from a HLA.A2 transgenic mouse, and treated with intraperitoneal injections of mMR16-1 or control antibody. DSA responses were monitored weekly for 5weeks by measurement of serum anti-HLA.A2 antibodies in a flow cytometric antibody binding assay. Results show that mMR16-1 significantly reduced DSA IgM, IgG2a and IgG1 responses, respectively, while normalizing serum amyloid A (SAA), an acute phase reactant induced by IL-6 (p<0.01 vs. control). mMR16-1 injections increased mononuclear cell apoptosis in the spleens, as detected by annexin V staining and TUNEL. In conclusion, anti-IL6R attenuates de novo DSA responses and suppresses inflammatory markers (SAA). The data indicate that antibody therapy targeting the IL-6/IL-6R pathway may serve as a strategy to suppress DSA generation.