The role of macrophages type 2 and T-regs in immune checkpoint inhibitor related adverse events

Immune checkpoint inhibitory (ICI) therapy represents a novel approach in a variety of cancers, with impressive survival benefit. With ICIs, however, a new spectrum of immune related adverse events (irAE) including life threatening hypohysitis has emerged. This autopsy study aimed to investigate inflammatory cells, PD-1 and PD-L1 expression in cases of patients who developed hypophysitis and involvement of other organs. We analysed 6 patients, who were treated with ICIs and developed hypophysitis. Two received an additional MAP-kinase inhibitor, MEK-inhibitor and cytotoxic chemotherapy. Besides the pituitary gland, all investigated adrenal glands (5/5) were affected; three cases had other organs involved (liver (2/6), thyroid (2/6), lung (1/6), myocardium (1/6), colon (1/6). The inflammatory cells of involved organs were further specified and PD1 and PDL-1 expression was analyzed using immunohistochemistry. We observed that patients treated with ICIs alone showed T-cell predominant lymphocytic infiltrates, whereas patients receiving additional therapies demonstrated an increase in B- and T-lymphocytes. Surprisingly, the dominant inflammatory population was not T-cell, but type 2 macrophages. CD25 positive T-regs were sparse or absent. Our study suggests that T cell activation is only partially responsible for irAE. ICI therapy interaction with CTLA-4, PD-1 and PDL-1 in type 2 macrophages appears to result in disturbance of their control. Furthermore, depletion of T-regs seems to contribute significantly. Our findings with simultaneous pituitary and adrenal gland involvement underlines the systemic involvement as well as the importance of monitoring cortisol levels to avoid potentially life threatening hypocortisolism.