Central role of leukemia-derived factors in the development of leukemia-associated immune dysfunction.

Abstract

The underlying mechanism of tumor-associated T-cell dysfunction has been suggested to be due to an acquired abnormality in the T-cell receptor signaling complex, however, tumor-derived inhibitory factors acting on immune effectors could also explain the observed immunosuppression. Here, in a murine acute leukemia model, responses (proliferation and IL-2 secretion) to mitogens were suppressed in the early stages of leukemic progression due to a leukemia-derived soluble factor(s). Correlations between the development of immunosuppression and changes in expression of TCR signaling proteins (CD3-zeta, p56(lck), p59(fyn)), calcium mobilization, and total tyrosine kinase activity following TCR stimulation were examined. In contrast to reduced responses to mitogens seen as early as 1 week following injection of leukemic cells, signaling abnormalities were only seen in advanced disease (ie four weeks). The only significant alteration in signaling protein expression was loss of CD3-zeta for four weeks following the initiation of leukemia. Importantly, a direct role of a leukemia-derived soluble factor(s) in the loss of CD3-zeta could be shown in vitro by co-culturing splenocytes with leukemic cells separated by a transwell for seven to 10 days. Further studies demonstrated that the leukemia-derived factor(s) stimulated splenic macrophages to secrete a second soluble factor(s) that caused the loss of CD3-zeta. These data indicate that soluble leukemia-derived factor(s) have both immediate immunosuppressive actions and a later effect acting indirectly to induce intrinsic defects in T-cell signaling pathways via accessory cells. In some tumors, there is evidence that the soluble factor is Fas-ligand, which induces apoptosis in tumor-infiltrating T cells and the action of activated caspases cleave CD3-zeta. In this model, the soluble factor prevents apoptosis indicating a second mechanism is responsible for the loss of T-cell signaling proteins.