Abstract
We have previously reported a novel phenotype of myeloid suppressors in lymphoma patients characterized by a loss of HLA-DR expression on monocytes, CD14(+)HLA-DR(low/neg). These cells were directly immunosuppressive and were associated with poor clinical outcome. In this study, we found that lymphoma tumors could have more than 30% of their tumor occupied by CD14(+) cells. This intimate spatial connection suggested substantial cell-cell communication. We examined cross talk between monocytes from healthy volunteers (normal) and lymphoma cells in co-culture to identify the mechanisms and consequences of these interactions. Normal CD14(+)HLA-DR(+) monocytes lost their HLA-DR expression after co-culture with lymphoma cells. Lymphoma-converted CD14(+)HLA-DR(low/neg) cells exhibited similar immunosuppressive functions as CD14(+)HLA-DR(low/neg) monocytes from lymphoma patients. Unexpectedly monocyte additions to lymphoma cell cultures protected lymphoma from cytotoxic killing by chemotherapy drug doxorubicin (DOX). Monocyte mediated resistance to DOX killing was associated with decreased Caspase-3 activity and increased anti-apoptotic heat shock protein-27 (Hsp27) expression. Soluble Hsp27 was detected in supernatant and patient plasma. Increased Hsp27 in plasma correlated with increased proportion of CD14(+)HLA-DR(low/neg) monocytes in patient blood and was associated with lack of clinical response to DOX. This is the first report to describe a non-immune function of CD14(+)HLA-DR(low/neg) monocytes: enhanced lymphoma resistance to chemotherapy. It is also the first report in lymphoma of Hsp27 as a potential mediator of lymphoma and monocyte crosstalk and chemotherapy resistance. Together with previous reports of the prevalence of these myeloid suppressors in other cancers, our findings identify this pathway and these interactions as a potential novel therapeutic target.