A UNIQUE IMMUNE SIGNATURE IN BLOOD SEPARATES THERAPY-REFRACTORY FROM THERAPY-RESPONSIVE ACUTE GRAFT-VERSUS-HOST DISEASE.

Acute Graft-versus-Host Disease (aGvHD) is an immune cell-driven, potentially lethal complication of allogeneic hematopoietic stem cell transplantation affecting diverse organs including the skin, liver and gastro-intestinal (GI) tract. We applied mass cytometry (CyTOF) to dissect circulating myeloid and lymphoid cells in children with severe (grade III-IV) aGvHD treated with immune suppressive drugs alone (1st line therapy) or in combination with mesenchymal stromal cells (2nd line therapy). These results were compared to CyTOF data generated in transplanted children with no aGvHD or age matched healthy controls. Onset of aGvHD was associated with appearance of CD11b+CD163+ myeloid cells in the blood and accumulation in the skin and GI-tract. Distinct T-cell populations, including TCRγδ+ cells, expressing activation markers and chemokine receptors guiding homing to the skin and GI-tract were found in the same blood samples. CXCR3+ T-cells released inflammation-promoting factors after overnight stimulation. These results indicate that lymphoid and myeloid compartments are triggered at aGvHD onset. IgM-, presumably class-switched, plasmablasts and two distinct CD11b- DC subsets were other prominent immune populations found early during the course of aGvHD in patients refractory to both 1st and 2nd line (MSC-based) therapy. In these non-responding patients, effector and regulatory T-cells with skin/gut homing receptors also remained proportionally high over time, while their frequencies declined in therapy responders. Our results underscore the additive value of high dimensional immune cell profiling for clinical response evaluation, which may assist timely decision making in the management of severe aGvHD.