Characterizing Immune Privilege in Hair Follicles: Could Somatostatin Play a Role?

Background:  Immune privilege (IP) is a phenomenon in which tissues are not attacked by the recipient immune system after histoincompatible transplantation. IP is observed in tissues such as the cornea, brain, and placenta. Unique mechanisms observed in the IP tissue can include decreased MHC Class I expression, impairment of antigen presenting cells, and secretion of immunoregulatory factors such as the neuropeptide somatostatin (SST). IP is believed to exist in the anagen hair follicle (HF). However, as there are few quantitative or functional studies examining IP in HFs, we wanted to examine the HF in more detail, our goal being to elucidate a functional mechanism of IP in HFs. Methods:  Human anagen HFs, obtained from plastic surgery procedures with ethics approval, were microdissected and the bulbs (lower third) and sheaths (middle third) were separated. We examined expression of 44 IP-related genes by quantitative RT-PCR (qPCR) in five HF and ten control epidermal samples. For several experiments, HF portions or epidermal tissues were processed into single-cell suspensions. Peripheral blood mononuclear cells (PBMC) from non-histocompatible healthy volunteers were co-cultured for 5 days with primary HF bulb, sheath, or epidermal cells. An IFN-γ ELISA was utilized to examine the relative PBMC activation response. Immunohistochemistry (IHC) was conducted to examine SST expression in human HFs. SST secretion in HF or epidermal cell cultures was detected by ELISA. To examine SST effect, PBMCs were cultured with allogeneic epidermal cells and varying concentrations of SST (16 to 1600 pg/ml), analyzing supernatant for IFN-γ. Results:  In co-culture, allogeneic HF sheath cells induced significantly less IFN-γ secretion (11.6 pg/ml) from PBMCs than epidermal cells (38.5 pg/ml). Briefly, by qPCR we found HLA Class I and Class II genes significantly downregulated in the HF, as well as immunosuppressive secretory factors that were significantly upregulated in the sheath region, including TGFβ2, αMSH, and MIF, consistent with previously published studies. Most notably, SST was significantly upregulated in the sheath by 5.9-fold and bulb by 94.2-fold compared to non-follicular skin epithelium. Examining SST further, by IHC, SST was strongly expressed in HF outer root sheath layers relative to epidermis. ELISA detected significantly greater SST secretion in sheath cell culture than epidermal or HF bulb cell culture. Additionally, we found that SST significantly inhibited PBMC IFN-γ secretion when stimulated by allogeneic epidermal cells, especially at lower concentrations. Conclusions:  Finding a functional mechanism of IP in HFs could have implications for new treatments for inflammatory hair loss diseases, such as alopecia areata, or for tissue transplantation. Our results reveal a novel finding that SST is expressed in the human HF and that it can have inhibitory effects on PBMC activation. At this point, it is unclear exactly what SST is doing for the HF, but we propose that it may have a role in HF IP.