ATG and Statins Reduce Incidence of Severe Chronic Gvhd By Distinct Mechanisms Involving CXCL9 and Kynurenine Catabolism

Introduction:

Severe chronic graft versus host disease (cGVHD) causes debilitating morbidity due to fibrotic and inflammatory changes in connective tissues mainly of skin, lungs, eyes and the gastrointestinal tract. In contrast, mild cGVHD is strongly associated with better overall survival due to lower relapse rates, so that permissive immunosuppressive drug management is actively pursued by many clinicians. Although alloreactive T lymphocytes are clearly involved in the induction of both grades of cGVHD, it is unpredictable which patients are prone to develop severe rather than mild chronic disease. Monokine induced by interferon gamma (MIG, CXCL9) has been correlated with cGVHD. CXCL9 is a member of the CXCR3 ligand family, which plays a role in other fibrotic diseases such as systemic sclerosis. Anthranilic acid (AA) reduced the severity of acute GVHD in mice. AA is the product of a complex metabolic pathway involving indoleamine 2,3-dioxygenase (IDO), (Trp), kynurenine (Kyn), and Vitamin B6. Based on our observation that anti-thymocyte globulin (ATG) and statins independently reduce the incidence of severe cGVHD, we investigated if MIG, IDO, Trp, Kyn and Vitamin B6 serum levels are correlated with cGVHD.

Patients and methods:

The incidence of cGVHD was evaluated in 554 patients who consented to participate in this observational study and who survived the first 6 months after alloSCT. Median age was 53 years, 333 were male. The underlying conditions were AML (157), ALL (49), MPN/ MDS (95), lymphoma (203), and multiple myeloma (99). Donors were matched related (194), matched unrelated (227), mismatched unrelated (113), or haploidentical (15). Myeloablative or aplasia conditioning was administered in 101, reduced intensity conditioning in 453 patients. 325 patients received ATG and 244 patients received pravastatin at a dose of 20 mg/d starting from day-1 of alloSCT as per institutional policy. 176 patients received both, ATG and pravastatin, whereas 159 patients received neither. Chronic GVHD was diagnosed and graded as severe or non-severe applying the National Institutes of Health’s 2005 consensus criteria.

Day +100 serum samples for measuring CXCL9, IDO, Trp and Kyn by ELISA were available for 350 patients and at onset of cGVHD for 185 patients. Furthermore, VitB6 was measured by HPLC on day +100 in 194 patients.

Results:

Chronic GVHD occurred in 295 patients (54%), 58 (11%) of whom developed severe cGVHD of skin, lungs or eyes, 40 (7%) isolated severe cGVHD of the gastrointestinal tract, and 197 (36%) developed non-severe cGVHD. ATG and statin were associated with reduced incidence of of severe cGVHD. In contrast, only ATG reduced mild chronic GVHD whereas statins did not (Figure 1). Increased levels of CXCL9 were observed for both mild and severe cGVHD at disease onset and on day+100 after alloSCT. In contrast, highest Kyn levels were measured at disease onset of patients with severe cGVHD of lung, skin and eyes. Patients who received ATG prior to transplantation showed significantly lower serum levels of CXCL9 at cGVHD onset, but not on d+100. In contrast, statins did not influence CXCL9 levels, but were associated with lower serum levels of Trp and Kyn and increased IDO levels on day+100. Higher Trp and Kyn serum levels on day+100 despite statin usage predicted higher incidence of severe cGVHD. The reduction of both, Trp and Kyn serum levels in the context of IDO activation suggested that Kyn catabolism is enhanced by statins. Indeed, higher Vitamin B6 serum levels compensated for statin failure and protected against severe cGVHD.

Conclusions: Ourdata suggest that ATG and statins minimize severe cGVHD by distinct mechanisms. The fact that ATG associates with reduced immune activation markers (CXCL9) at cGVHD onset, but has no impact on homeostatic serum markers on day +100 is consistent with initial depletion of alloreactive T cells during the early post-transplant period, resulting in weaker immune activation after tapering immunosuppressive therapy. In contrast, statin intake seems to induce IDO and reduce Trp serum levels on day+100 along with activation of Kyn catabolism, pointing to a pathophysiological role of this pathway in the prevention of cGVHD. Accordingly, Vitamin B6 levels could reduce severe cGVHD incidence in patients who failed to lower Trp and Kyn levels in the context of statins.

The synergism of statin and Vitamin B6 is clinically relevant and warrants further studies.