PD-L1 expression and response to immune checkpoint inhibitor therapy in malignant histiocytic neoplasms

Malignant histiocytic neoplasms (MHN) are aggressive cancers without an established effective treatment. Recent studies have suggested possible role of immune checkpoint inhibitors (ICI), but predictors of response are unknown. We analyzed 26 MHNs to determine the frequency of PD-L1 expression and clinicopathologic correlates. PD-L1-assessment on tumor cells was manually scored negative or positive (≥1%). Median age at diagnosis was 59 years and PD-L1-positivity was seen in 73%. Those with ≥50% PD-L1 expression had better overall survival compared to PD-L1 < 50% (100% vs 42%; P-value:0.0324). The overall response rate (ORR) to ICI in six MHNs was 50%. A comprehensive literature review revealed ORR of 64% in 14 MHN patients treated with ICI. Combining patient and literature cohorts treated with ICI (<i>N</i> = 20), responders showed a higher median PD-L1 expression compared to non-responders (88% vs 30%; <i>P</i>-value: 0.0737). While ICI is promising in MHN-subset, larger cohorts need to be systematically investigated to analyze the long-term sustainability and efficacy with/without combination chemotherapy/radiation.