The JAK1/2 Inhibitor Ruxolitinib Downregulates the Immune Checkpoint Protein B7-H3 in Multiple Myeloma

Introduction: The JAKSTAT pathway plays a critical role in the regulation of hematopoietic pathways and immunological cytokine signaling. The JAK pathway is also involved in tumor cell proliferation and drug resistance in multiple myeloma (MM). Thus, inhibition of the JAK pathway should be a potentially effective strategy for treating MM patients. B7-H3 is an immune checkpoint protein in the B7 superfamily and has been shown overexpressed in several tumors. Immune checkpoint blockade may suppress tumor progression or enhance anti-tumor immune responses. In this study, we investigated the effects of the JAK1/2 inhibitor ruxolitinib (Rux) on B7-H3 in MM.

Materials and Methods: Bone marrow mononuclear cells (BMMCs) were collected from MM patients after obtaining IRB approval. Single-cell suspensions were prepared from human MM LAGλ-1A xenografts which had been grown in severe combined immunodeficient mice. HS-5 stromal and SUP-T1 T cells were purchased from ATCC. The cells were cultured and treated with or without RUX and then subjected to qRT-PCR, flow cytometric analysis, and western blot analysis. For qRT-PCR, total RNA was extracted and applied to cDNA synthesis, followed by qPCR. Gene expression was analyzed in MM BMMCs alone or co-cultured with stromal cells or T cells with or without Rux treatment (1μM) in vitro.

Results: We identified increased B7-H3 expression in MMBMMCs from patients with progressive disease (PD) patients compared to those in complete remission (CR). Rux significantly reduced B7-H3 expression in MMBMMCs in patients with PD, MM cells (U266), and BM from patients in PD when co-cultured with stromal cells (HS-5) after 48-72 hours. Rux decreased B7H3 expression in the human MM xenograft model LAGλ-1A when cultured ex vivo. In addition, Rux suppressed B7-H3 at protein levels as shown with flow cytometric analysis and western blotting, consistent with the gene expression results.

Next, we tested whether B7-H3 blockade by Rux could potentially restore exhausted T cell activity against myeloma cells in MMBM. We found that Rux can increase IL-2 and CD8 gene expression in MMBM with lower plasma percentages (< 30%) but not among those with higher plasma cell percentages (>70%). Rux also elevated IL-2 and CD8 gene expression in BM when it was cocultured with T cells (SUP-T1), suggesting Rux may mediate immunological cytokine signaling. B7-H3-neutralizing antibody increased CD8 gene expression in MMBM in vitro, suggesting that one of the mechanisms through which Rux upregulates CD8 T cells in MMBM may be via downregulation of B7-H3.

Conclusion: The immune checkpoint protein B7-H3 is overexpressed in MMBM in PD compared to CR patients. The JAK1/2 inhibitor Rux can decrease B7-H3 expression and increase IL-2 and CD8 expression in BM in vitro. Our results provide evidence for Rux inhibiting the immune checkpoint protein B7-H3 which may potentially restore exhausted T-cell activity in the MMBM tumoral microenvironment.