Paracrine regulations of IFN-γ secreting CD4 T cells by lumican and biglycan are protective in allergic contact dermatitis.

Allergic contact dermatitis (ACD) is a delayed-type IV hypersensitivity response driven by innate and adaptive immune cells. While specific immune regulations of these cell types are amply elucidated, their regulations by extracellular matrix (ECM) components and T cell mediated adaptive immunity in ACD remains unclear. Lumican and biglycan are ECM proteoglycans abundant in the dermis and lymph node, known to regulate innate immune myeloid cells, but have not been investigated in lymphoid cell regulations in ACD. By immunohistology we localized lumican and biglycan in skin biopsies of psoriatic patients. Using wild type (WT), lumican and biglycan knockout mice, we investigated CD4T cell infiltration, activation and proliferation in the skin and draining lymph node (dLN) of contact hypersensitivity (CHS)-challenged mice by immunohistochemistry and flow cytometry. We used the OT-II adoptive transfer model to test antigen specific CD4T cell activation. We assessed interactions of the proteoglycans with LFA-1 on T cells by confocal microscopy. Compared to WTs, the knockouts showed severe ear inflammation, with increased CD4T cells infiltration in the dermis. CHS-challenged knockout mice dLN showed increased T-bet, STAT1 and -STAT4 signaling, indicating enhanced Th1 commitment and proliferation. We found that WT lymph node fibroblastic reticular cells (FRCs) secrete lumican, biglycan and decorin, a related proteoglycan, while none are expressed by naive or activated T cells. Lumican and biglycan interact with LFA-1 on T cell surfaces, and in vitro all three proteoglycans suppress CD4T cell activation. Secreted by dLN FRCs, lumican, biglycan, and possibly decorin interact with LFA-1 on CD4T cells to restrict their activation and reduce dermatitis severity.