Combination immunotherapy with anti-PD-L1 antibody and depletion of regulatory T cells during acute viral infections results in improved virus control but lethal immunopathology.

Combination immunotherapy (CIT) is currently applied as a treatment for different cancers and is proposed as a cure strategy for chronic viral infections. Whether such therapies are efficient during an acute infection remains elusive. To address this, inhibitory receptors were blocked and regulatory T cells depleted in acutely Friend retrovirus-infected mice. CIT resulted in a dramatic expansion of cytotoxic CD4+ and CD8+ T cells and a subsequent reduction in viral loads. Despite limited viral replication, mice developed fatal immunopathology after CIT. The pathology was most severe in the gastrointestinal tract and was mediated by granzyme B producing CD4+ and CD8+ T cells. A similar post-CIT pathology during acute Influenza virus infection of mice was observed, which could be prevented by vaccination. Melanoma patients who developed immune-related adverse events under immune checkpoint CIT also presented with expanded granzyme-expressing CD4+ and CD8+ T cell populations. Our data suggest that acute infections may induce immunopathology in patients treated with CIT, and that effective measures for infection prevention should be applied. Author summary Combination immunotherapy (CIT) directed against checkpoint mechanisms has been approved for the therapy of cancers and is proposed for the treatment of chronic infections. In cancer therapy patients often develop severe immunopathology under CIT. Here we show that acute viral infections (Friend retrovirus and Influenza virus) posed a significant threat during CIT in mice. The strong activation of cytotoxic T cells after an acute viral infection seemed to lack couterregulation during CIT, which resulted in lethal immunopathology. In case of an Influenza virus infection this could be prevented by vaccination prior to CIT. The expansion of CD4+ and CD8+ T cells expressing cytotoxic molecules were also observed in melanoma patients treated with two checkpoint blockers. Thus, these patients might also be at risk of developing severe immunopathology during an otherwise harmless acute viral infection. This should to be taken into account when cancer patients are undergoing CIT.