Marrow-Infiltrating T Cells In Patients with Chronic Lymphocytic Leukemia Display Markers of Functional Impairment and Express PD-1

Abstract 2417 Marrow is a major site of disease development and progression for chronic lymphocytic leukemia (CLL), as well as a priming site for antigen-specific T cells and a reservoir for memory T cells. To determine the extent to which T cells in the marrow microenvironment have an altered phenotype and function in CLL, we analyzed the immunophenotypic characteristics of marrow-infiltrating T cells of 18 CLL patients compared to 11 normal donors. Chemotherapy-naive CLL patients (n=7) possessed comparable quantities of marrow T cells compared to normal donors (median CD8+ T cells/μl = CLL 904 vs normal 1247; median CD4+ T cells/μl = CLL 1975 vs normal 1110). However, we identified several aberrant characteristics among T cells infiltrating the marrow of CLL patients. First, the ratio of CD8+ to regulatory T cells (CD4+CD25+FOXP3+) was depressed (median ratio CLL 14 vs normal 41), indicating more regulatory T cells per effector T cells in CLL. Second, compared to normal marrow T cells, CLL marrow contained proportionally fewer functional effector CD8+ T cells (CD27+CD28+)(median normal 57%, CLL 48%) and more immunosenescent cells (CD27-CD28-)(median normal 21%, CLL 30%). Third, the T cell differentiation state of CLL CD8+ T cells was skewed to favor a phenotype of increased terminal differentiation (CD45RA+CCR7-)(median CLL 55% vs normal 40%), and decreased naive (CD45RA+CCR7+) cells (median CLL 21% vs normal 31%) compared to normal donors. These differences were further accentuated in CLL samples collected within 4 months from treatment with conventional chemotherapy (n=11). Finally, by immunohistochemical staining of CLL marrow biopsies, we observed marrow-infiltrating lymphocytes to express PD-1 (mean of infiltrating T cells, untreated CLL 12%, treated CLL 35%, present even >6 months after therapy), a marker associated both with immuno-activation and inhibition. While the majority of PD-1+ CD8 T cells of normal donors (n=5) and treated CLL patients (n=4) were differentiated towards effector memory (CD45RA-CCR7-) cells (median normal 46% vs untreated CLL 16%, p=0.07; treated CLL 61%), the PD-1+ T cells from untreated CLL patients (n=5) were terminally differentiated (CD45RA+CCR7+)(median normal 23% vs untreated CLL 65%, p=0.04; treated CLL 24%). These results indicate an exhausted rather than an activated T cell phenotype in untreated patients. Paired immunophenotypic analysis on blood and marrow from the same individuals (n=9) demonstrated an increased percentage and intensity of PD-1 expression on T cells from marrow compared to blood (percentage CD8+ T cells BM vs blood p = 0.05). Interestingly, PD-1 was also detected on CLL cells (n=16) but not normal B cells (median normal 0%, vs CLL 17%, p = 0.004). The ligand for PD-1, PD-L1, was detected in the marrow vasculature by immunohistochemical staining of biopsies, suggesting that the marrow microenvironment plays a role in the induction of PD-1 associated immunosuppression. Ligation of blood PD-L1 on CLL-T cells led to a 2-fold decrease in activation (measured as CD69 expression) of CD3/CD28 stimulated patient T cells. In summary, we identify several phenotypic and functional alterations within marrow-infiltrating T cells of CLL patients. We speculate these together may contribute to impaired priming of host immunity against the tumor. The PD-1 pathway appears to be activated in CLL, especially in the setting of chemotherapeutic treatment. Since anti-PD1 antibodies are now clinically available, it may be possible to target this pathway to improve anti-tumor responses. Disclosures: No relevant conflicts of interest to declare.