Elevated Serum sIL-2Ra Levels Facilitate IL-2 Signaling and Contribute to Impaired Tumor Immunity in B-Cell Non-Hodgkin Lymphoma (NHL).

Abstract 281

Elevated serum levels of soluble interleukin-2 receptor alpha (sIL-2Ra) have been shown to be a poor prognostic factor in various malignancies and sIL-2Ra levels have generally correlated with tumor bulk and advanced stage of disease. In a clinical trial of single agent rituximab in previously untreated follicular lymphoma (FL) patients, we found that sIL-2Ra levels were elevated compared to controls and increased sIL-2Ra levels prior to treatment were associated with a poor outcome. The mean sIL-2Ra level (+/− standard deviation) in untreated FL patients (n=32) was 2.13 ng/ml (+/− 1.81) and 1.09 ng/ml (+/− 1.53) for normal controls (n=16). The time to progression for previously untreated FL patients after 4 doses of rituximab was 12 months for patients with sIL-2Ra levels above the mean compared to 40 months for patients with low sIL-2Ra levels (p=0.003) confirming that elevated sIL-2Ra is a poor prognostic factor in FL. To explore the mechanism by which sIL-2Ra may contribute to a poor prognosis, we determined the source of sIL-2Ra and whether sIL-2Ra facilitates interleukin-2 (IL-2) signaling. Based on our previous work showing that IL-2 induces Foxp3 expression, we particularly evaluated whether sIL-2Ra may bind IL-2 and contribute to impaired anti-tumor immunity by promoting T_reg cell development.

We found that sIL-2Ra could be detected by ELISA in the culture medium of IL-2Ra-expressing cell lines such as SuDHL1 and Karpas299 and that activation of these cells with PMA/Ionomycin significantly increased sIL-2Ra levels in the culture medium. Production of sIL-2Ra was associated with an attenuation of surface IL-2Ra expression on SuDHL1 and Karpas299 cells suggesting that surface IL-2Ra is the source of sIL-2Ra. Next, using histo-tagged sIL-2R, we tested whether the complex between sIL-2Ra and IL-2 was able to bind to IL-2R on the malignant cell. We found that sIL-2Ra bound IL-2 and that the complex could be detected on the surface of IL-2R-expressing cells. We then compared the signaling of the sIL-2Ra/IL-2 complex to that of IL-2 alone. As expected, we found that IL-2 stimulated cell proliferation, enhanced STAT5 phosphorylation and upregulated Foxp3 expression in CD4⁺ T cells in a dose-dependent manner. Administration of an anti-IL-2 antibody attenuated these effects. In contrast, sIL-2Ra alone had little effect on cell proliferation, STAT5 phosphorylation and Foxp3 expression. However, the addition of sIL-2Ra to IL-2 prior to incubation with IL-2R-expressing cells enhanced IL-2-mediated phosphorylation of STAT5 and Foxp3 expression in CD4⁺ T cells. Compared to IL-2 alone, treatment with sIL-2Ra (25ng/ml) and IL-2 (25ng/ml) increases the number of CD4⁺ T cells with phosphorylation of STAT5 from 7.9% to 17.4%.

Taken together, these results indicate that sIL-2R plays an active biologic role in FL by binding IL-2 and promoting IL-2 signaling rather than depleting IL-2 and blocking its function. Given the findings that IL-2 strongly induces Foxp3 expression and promotes T_reg cell development and that T_reg cells suppress anti-tumor immunity, we conclude that elevated serum levels of sIL-2Ra facilitate IL-2 signaling and thereby contribute to a poor prognosis in FL.