Peripheral Blood T‐Cell Responses to Keratin Peptides that Share Sequences with M Proteins are Largely Restricted to Skin‐Homing CD8+ T Cells

Abstract

The association of psoriasis with throat infections by streptococcus pyogenes suggests a potential antigenic target for the T cells that are known to infiltrate dermis and epidermis of psoriatic skin. Streptococcal M protein shares an extensive sequence homology with human epidermal keratins. Keratins 16 (K16) and 17 (K17) are mostly absent from uninvolved skin but are upregulated in psoriatic lesions. There is increasing evidence that CD8+ T cells play an important effector role in psoriasis and M protein‐primed T cells may recognize these shared epitopes in skin via molecular mimicry. To identify candidate epitopes, peptides with sequences from K17 were selected on the basis of predicted binding to HLA‐Cw6 and sequence similarities with M6 protein. Matched peptides from the sequence of M6 protein and a set of peptides with poor predicted binding were also selected. Cw6+ individuals with psoriasis and Cw6+ healthy controls, having a family history of psoriasis, were recruited. PBMCs were incubated with the peptide antigens. T‐cell activation in the CD4+, CD8+ and later the skin‐homing cutaneous lymphocyte‐associated antigen (CLA)‐expressing subset of CD8+ T cells was evaluated by CD69 expression and intracellular IFN‐γ accumulation using flow cytometry. We demonstrate that Cw6+ psoriasis patients had significant CD8+ T‐cell IFN‐γ responses to peptides from K17 and M6 protein selected on the basis of sequence homology and predicted HLA‐Cw*0602 binding. These responses were about 10 times more frequent in the skin‐homing cutaneous lymphocyte‐associated antigen‐expressing (CLA+) subset of CD8+ T cells. CD4+ T cells showed only borderline responses. CD8+ T cells from Cw6 + nonpsoriatic individuals responded to some M6 peptides but very rarely to K17 peptides, and this also applied to the CLA+CD8+ subset. These findings indicate that psoriatic individuals have CD8+ T cells that recognize keratin self‐antigens and that epitopes shared by streptococcal M protein and human keratin may be targets for the CD8+ T cells that infiltrate psoriatic skin lesions.