Immune cell subsets in interface cutaneous immune-related adverse events associated with anti-PD-1 therapy resemble acute graft versus host disease more than lichen planus

Background: Checkpoint immunotherapy is frequently associated with cutaneous immune-related adverse events (cirAEs), and among those, the most common subtype shows interface reaction patterns that have been likened to lichen planus (LP); however, cutaneous acute graft versus host disease (aGVHD) may be a closer histopathologic comparator. We used quantitative pathology to compare the immunologic composition of anti-PD-1-associated interface reactions to LP and aGVHD to assess for similarities and differences between these cutaneous eruptions. Methods: Immunohistochemistry for CD4, CD8, CD68, PD-1, and PD-L1 was performed on formalin-fixed paraffin-embedded tissue from patients with anti-PD-1 interface cirAEs (n = 4), LP (n = 9), or aGVHD (n = 5). Densities of immune cell subsets expressing each marker were quantified using the HALO image analysis immune cell module. Plasma cell and eosinophil density were quantified on routine H&E slides. Results: Specimens from patients with anti-PD-1 interface cirAEs showed equivalent total cell densities and immune cell composition to those with aGVHD. Patients with LP showed higher total immune cell infiltration, higher absolute T-cell densities, increased CD8 proportion, and reduced histiocytic component. The cases with the highest plasma cell counts were all anti-PD-1 interface cirAEs and aGVHD. Conclusion: The composition of immune cell subsets in anti-PD-1 interface cirAEs more closely resembles the immune response seen in aGVHD than LP within our cohort. This warrants a closer look via advanced analytics and may have implications for shared pathogenesis and potential treatment options.