Retrospective Evaluation of Clonality in Canine Erythema Multiforme.

Background: Erythema multiforme (EM) and similar cytotoxic dermatoses, including Stevens-Johnson syndrome (SJS) and toxic epidermal necrolysis (TEN), represent immune-mediated conditions that have clinical, histopathological and immunohistochemical overlap with other diseases. Although reactive processes are typically associated with polyclonal expansion of lymphocyte populations, benign clonal expansion is possible in non-neoplastic conditions.

Hypothesis/objectives: The purpose of this study is to elucidate the role of clonality in differentiating cases of canine EM/SJS/TEN from cutaneous epitheliotropic lymphoma. Further aims include providing clinical correlation and response to therapy in combination with clonality. It is hypothesised that both clonal and polyclonal expansions will be observed in cases of EM/SJS/TEN.

Animals: Twelve dogs with clinical and histopathological changes supportive of EM or SJS/TEN.

Materials And Methods: Clinical data, histological and immunohistochemical examination as well as clonality for T-cell receptor gamma (TRG) was performed for tissue samples in canine EM/SJS/TEN. Modified drug scoring was performed for cases with medication administration before lesion development.

Results: Twelve cases were included for retrospective review. Good response to therapy, CD3 immunoreactive T cells, and at least minor expression of Granzyme B were noted in all cases. Eleven of 12 had mild-to-moderate CD20 dermal infiltration. Polyclonal populations were noted in four cases, polyclonal with minor clones in five cases and clonality in three cases. Modified drug scoring was positive in five of six cases.

Conclusions And Clinical Relevance: This study describes cases of canine EM/SJS/TEN demonstrating both polyclonal and clonal T-cell expansion, further highlighting the need for pairing clinical response with histopathological results and advanced diagnostics.