Shirouchi K, Koshikawa S, Shinya K, et al. - In the present study, the researchers sought to clarify the role of programmed cell death 1 (PD-1) and PD- L1 in the pathogenesis of lichen planus in the absence of immune checkpoint inhibitors administration, using biopsy specimens. Immunohistochemistry of PD-1 and PD-L1 in 12 cases each of lichen planus and dermal-type erythema multiforme was conducted. The expression of PD-1 and PD-L1 on infiltrating inflammatory cells, predominantly lymphocytes, was significantly lower in lichen planus than in dermal-type erythema multiforme. The findings suggest that decreased PD-1 and PD-L1 expression may play a role in accelerating inflammatory cell infiltration into the epidermis during the pathogenesis of lichen planus.