Role of Monocytes and Pro-Inflammatory Th17 Cells Expressing Multi-Drug Resistance Protein Type 1 in the Pathogenesis of Graft-Versus-Host Disease

Graft-versus-host disease (GvHD) is a major cause of morbidity and mortality after allogeneic hematopoietic cell transplantation (HCT). The pathophysiology of GvHD is complex and remains poorly understood. However, it has been demonstrated that Th17 cells play an important role in the development and progression of acute and chronic GvHD. Furthermore, monocytes have an essential function in the induction of Th17 cells. It is known that the distribution of the three monocyte subtypes (classical (CD14⁺⁺CD16^-), non-classical (CD14⁺CD16⁺⁺) and intermediate (CD14⁺⁺CD16⁺) monocytes) changes significantly in the peripheral blood of patients during GvHD: the level of classical monocytes is decreased, whereas the percentages of intermediate and non-classical monocytes are elevated. Additionally, there is first evidence that intermediate monocytes are expanded during rheumatoid arthritis and promote the expansion of Th17 cells. Recently it could be demonstrated that pro-inflammatory Th17 cells are restricted to a novel subset of CCR6⁺CXCR3^hiCCR4^loCCR10^-CD161⁺ cells that stably express P-glycoprotein/multi-drug resistance protein type 1 (MDR1) – the so-called Th17.1 cells. These MDR1⁺Th17.1 cells are enriched and activated in the gut of patients with Crohn´s disease and are resistant to glucocorticoid-mediated T cell suppression. Thus, this subtype of Th17 cells might play an important role in steroid-resistant inflammatory diseases as well as steroid-refractory GvHD.

Our previously performed study could demonstrate that monocytes isolated from patients with acute or chronic GvHD grade I-IV (n=13) induced significant higher percentages of IL-17-producing Th17 cells compared to monocytes from healthy donors (n=32) or patients without GvHD after HCT (n=21) (p<0.001).  To further investigate the role of monocytes in the pathogenesis of GvHD the influence of classical, non-classical and intermediate monocytes on the induction of pro-inflammatory MDR1⁺CCR6⁺CXCR3^hiCCR4^loCCR10^-CD161⁺ Th17.1 cells was investigated in the present study. Therefore, these monocyte subtypes were isolated from peripheral blood mononuclear cells (PBMCs) by magnetic cell isolation technology and were co-cultured with isolated CD4⁺ T cells. The obtained results could show that intermediate and non-classical monocytes induced higher percentages of pro-inflammatory MDR1⁺CCR6⁺CXCR3^hiCCR4^loCCR10^-CD161⁺ Th17.1 cells compared to classical monocytes or whole monocyte population. Additionally, the influence of glucocorticoids used for the treatment of patients with GvHD and the Hsp90 inhibitor 17-Dimethylaminoethylamino-17-demethoxygeldanamycin (17-DMAG) was examined on the development of MDR1 expressing Th17.1 cells in vitro. Therefore, monocytes were co-cultured with CD4⁺ T cells in the presence and absence of prednisolone, dexamethasone and 17-DMAG. First data demonstrated that treatment of these co-cultures with glucocorticoids resulted in elevated levels of induced pro-inflammatory MDR1⁺Th17.1 cells. In contrast, addition of the Hsp90 inhibitor 17-DMAG to the co-cultures led to decreased levels of these pro-inflammatory Th17 cells compared to the untreated control.

In conclusion, our findings suggest that intermediate and non-classical monocytes trigger the induction of pro-inflammatory MDR1⁺CCR6⁺CXCR3^hiCCR4^loCCR10^-CD161⁺ Th17.1 cells and might therefore play a major role in the pathogenesis of GvHD. While glucocorticoids seem to promote the development of MDR1 expressing Th17.1 cells, the Hsp90 inhibitor 17-DMAG acts anti-inflammatory leading to decreased levels of induced pro-inflammatory Th17 cells. Thus, the chaperone Hsp90 could be an important regulator in monocyte-induced development of pro-inflammatory MDR1⁺Th17.1 cells and might therefore be a therapeutic target for GvHD.