Nibling of MHC Class II by Tfh Cells—Possible Role in Isotype Switching and Affinity Maturation

IntroductionTrogocytosis is an intracellular transfer of membrane and membrane-associated proteins in a cell–cell contact manner. Trogocytosis plays an important role in immune regulation. We hypothesized that trogocytosis may play role in isotype switching and affinity maturation in the germinal center.Trogocytosis of MHC-II by T follicular helper (Tfh) (CD4+CXCR5+CD45RA-) from dendritic cells (DCs) results in MHC-II-dressed Tfh to present antigen and stimulate B cells, resulting in B cell activation and proliferation with an important role in isotype switching and affinity maturation.Methodology:Monocyte-derived DCs (moDCs) were prepared by culturing purified CD14+ monocytes isolated from peripheral blood mononuclear cells (PBMCs), enriched by immunomagnetic positive selection and addition of GM-CSF (50 ng/ml) and IL-4 (20 ng/ml) for 6 days. Purified CD4+T cells were enriched by immunomagnetic negative selection and were activated using anti-CD3/CD28 beads for 24 hours. MoDCs were co-cultured with activated CD4+T cells (1:1) for 4 hrs. Harvested co-cultured cells were stained with antihuman monoclonal antibodies and isotype controls. The cells were washed and acquired on FACS Calibur and analyzed using FlowJo.Expression of MHC-II, co-stimulatory molecule (CD86), and mean fluorescence intensity (MFI) was measured on activated CD4+T, Tfh, and their subpopulations. Activated CD4+T cells and moDCs without co-culture served as controls. Statistical analysis was performed by a parametric t test, and a value of p<0.05 was considered significant.ResultsOur data show that following trogocytosis, MHC-II expression was significantly increased on activated CD4+T (p = 0.002), Tfh (p = 0.0002), Tfh1 (p = 0.0002), Tfh2 (p = 0.0001), and Tfh17 (p = 0.0004) cells. The MFI was also higher on total CD4+T (p = 0.01), Tfh (p = 0.0007), Tfh1 (p = 0.001), and Tfh2 (p = 0.03). The expression and MFI of CD86 were higher on total CD4+T, Tfh cells, and its subsets.ConclusionTrogocytosis results in MHC-II-dressed Tfh cells with Tfh1 showing a higher proportion of trogocytosed class MHC-II, followed by Tfh17 and Tfh2. Also, there is trogocytosis of CD86 on CD4T, Tfh, and its subsets. Future directions involve the exploration of the functional role of MHC-II-dressed Tfh cells as potent APCs and their contribution to isotype switching, affinity maturation, and restoration of immune function in patients with inborn errors of immunity.