Altered Naive and Memory T Cell Homeostasis and Immunosenescence Characterize Younger Patients with Immunosuppressive-Responsive Myelodysplastic Syndrome

BACKGROUND: A subset of patients with Myelodysplastic Syndrome (MDS) responds well to immunosuppressive therapy (IST) and the only validated predictor of response is age, with younger patients faring much better than older patients. Hematologic improvement on immunosuppressive therapy is associated with a survival benefit with response rates ranging from 15% to 50%, clearly comparable or better than results with other existing therapies in MDS. Despite progress in the basic understanding of immune pathobiology of MDS and a clear therapeutic value, including improved long-term survival, IST including anti-thymocyte globulin (ATG) and/or cyclosporine A (CyA) is rarely offered to MDS patients in the U.S. due to uncertain criteria for selection of patients and potential toxicities. In addition, there is an underlying concern that inappropriate use of immunosuppressive therapy may negatively impact risk for leukemia progression, which occurs in 30–40% of MDS cases. The long-term goal of this study is to identify an immune signature that has postive predictive power for IST responsiveness.

METHODS: To determine the effect of age on T-cell homeostasis and function and IST response, we performed a study of 54 MDS patients compared to 37 healthy controls. In a pilot study, T cell abnormalities associated with response to equine anti-lymphocyte globulin (eATG, lymphoglobulin, Pfizer, Inc) and/or CyA was studied in 12 younger MDS patients composed of 6 responders and 6 non-responders.

RESULTS: CD4+ T-cells are normally present in the peripheral blood lymphocyte pool at 2 to 4 times greater than that of CD8+ T-cells, and diminished CD4:CD8 ratio has been previously shown to correlate with poor survival outcome in MDS. Similar to previous reports, we found that the age-adjusted CD4:CD8 ratio was reduced in MDS patients compared to healthy controls (p-value <0.0001) Interestingly, our analysis revealed that inadequate CD4+ rather than expansion of CD8+ T-cells was associated with a lower ratio in this group of MDS patients that included both lower and higher risk MDS patients defined by the International Prognostic Scoring System (IPSS). Analysis of the percentage of T-cells with naïve and memory phenoytpes using CD45RA and CD62L display, demonstrated positive correlations between age and both % CD62L positive naïve cells and central memory CD4+ T-cells (naïve: slope=0.39, p=0.12; central memory: slope=1.26, p=0.005). Furthermore, the proportions of CD62L- CD4+ T-cell populations, including effector memory and terminal effector memory T-cells, were greater in younger MDS patients (slope=−0.82, p=0.08 and slope=−0.83, p=0.015, respectively) suggesting a possible relationship to IST responsiveness. Specific characteristics associated with response to eATG in the pilot study of 12 younger patients included altered distribution of T cell populations (i.e., lower CD4/CD8 ratio, p<0.001) and higher constitutive proliferative index of the T cell populations (p=0.03 CD4+ and p=0.02 CD8+ T-cells, respectively). We also found that hematological response was associated with blockade of homeostatic proliferation of T cells associated with reconstitution of the naïve T cell pool. Reduction in CD4+ T-cells and expansion of autoreactive CD8+ T-cells suggests that apoptotic conditions may drive the expansion of cells through homeostatic cytokines such as IL-7, IL-15, and/or IL-21, which are all cytokines of the IL-2Rγ_c family that control homeostatic proliferation. Comparisons of the IL-7Ra, IL-15Ra, IL-2Ra, and IL-21Ra subunit demonstrated overexpression of IL-21Ra in patients 35.4% ± 3.4 in CD4+ T-cells and 31.8% ± 4.3 in CD8+ T-cells compared to healthy donors 0.9% ± 0.5 and 0.5% ± 0.5 (p<0.0001).

CONCLUSIONS: Association between the T-cell abnormalities reported in this study and response to IST strongly suggests that aberrant T-cell homeostasis may represent a critical determinant of autoimmunity in MDS that may have positive predictive power for response to IST.