Mechanisms of immune escape and extramedullary tropism in leukemia cutis Available to Purchase

• Leukemia cutis is infiltrated by T cells with expression signatures of T cell exhaustion, in contrast to bone marrow-involved AML

• Tropism of leukemia to skin sites associates with altered expression of homing molecules and down-regulation of MHC class II

The mechanisms that lead to extramedullary tropism of acute myeloid leukemia (eAML) remain obscure and no specific therapeutic approaches for this entity exist. As the long-term survival of eAML is poor, a deeper understanding of the immune microenvironment and leukemia phenotypes underlying this entity is warranted. Here, we performed bulk and single-cell transcriptome profiling of 23 eAML biopsies from 10 patients with isolated extramedullary disease in skin and subcutaneous tissue. Unlike normal healthy skin, we found leukemia cutis to be heavily immune-infiltrated; in cases of extramedullary relapse following allogeneic stem cell transplantation, >90% of T/NK cells were donor-derived. eAML-associated T cells expressed a clear signature of T cell exhaustion, dissimilar to leukemia-associated immune populations in bone marrow relapse (n=7), but related to acute and chronic skin inflammation. Further, HLA class II was down-regulated in 4 of 7 leukemia cutis specimens, consistent with an immune escape phenotype in cases of eAML. Extramedullary and bone marrow-resident leukemia cells differed with regard to the expression of 8 homing receptor molecules (ICAM1 (encoding CD54), PECAM1 (CD31), ITGA4, ITGA6, ITGAL, ITGB4, ITGA5, and ITGAV). Serial samples obtained from one leukemia cutis case throughout consecutive immune checkpoint blockade with ipilimumab followed by nivolumab showed a consistently high degree of overlap between local and circulating T cell receptor (TCR) sequences, suggesting that only a minority of eAML-associated T cells are leukemia-specific. Our analysis reveals eAML to associate with complex changes in leukemia and T cell gene expression profiles that suggest multiple potential avenues for therapeutic targeting.