Human mucosal CD4+T cells but not blood CD4+T cells respond vigorously towards CD28 engagement

Human lamina propria T lymphocytes (LPT) possess functional properties profoundly different from those of peripheral blood T lymphocytes (PBT). While they are characterized by a low proliferative response to T cell receptor (TCR)/CD3 stimulation in vitro their responsiveness to activation through the co-stimulatory CD2-receptor is enhanced when compared to PBT. In this study, we demonstrate that engagement of another co-stimulatory receptor on both LPT and PBT, namely CD28, by a single monoclonal antibody (mAb), respectively, strongly activates the former but not the latter through a PI3-kinase dependent signalling pathway leading to the production of inflammatory cytokines such as interleukin (IL)-2, tumour necrosis factor (TNF)-alpha, interferon (IFN)-gamma and granulocytemacrophage colony-stimulating factor (GM-CSF). In addition to the high sensitivity of LPT to CD2 stimulation, this finding supports the notion that non-specific/innate mechanisms to activate T lymphocytes play a predominant role vis-a-visTCR driven/adaptive responses in the intestinal mucosa. Furthermore, it suggests that results from preclinical tests for therapeutic antibodies performed with human blood derived T cells are probably insufficient to predict reactivities of tissue-resident immune cells, which given their quantitative predominance may critically determine the in-vivo response to such compounds.