The CXCR4/STAT3/IL-10 Pathway Controls the Immunoregulatory Function of Chronic Lymphocytic Leukemia (CLL) and Can be Modulated By Lenalidomide

Patients with CLL experience generalized immune suppression, susceptibility to infections and secondary malignancies that likely involve complex bi-directional interactions between leukemic cells, components of the tumor microenvironment and immune effectors. CLL cells are capable of secreting IL-10 and exhibit regulatory functions comparable to that of normal B10 cells, a regulatory B cell subset that suppresses effector T-cell function through STAT3-mediated production of IL-10. However, the underlying mechanisms governing IL-10 production by CLL cells are not fully understood.

The chemokine CXCL12 is constitutively secreted by bone marrow stroma in CLL, and binds CXCR4 to direct chemotaxis, support tumor survival and activate various signaling pathways, including STAT3. Thus, we investigated if CXCL12 can enhance IL-10 production by activating the STAT3 pathway in CLL. Using peripheral blood mononuclear cells (PBMC) from 24 CLL patients who had not received therapy for ≥2 years, we showed that CXCL12 can enhance IL-10 production by CLL cells by activating S727-STAT3. This effect was CXCR4-mediated since blocking the CXCR4-CXCL12 interaction with a blocking antibody abolished CXCL12-induced IL-10 production. Addition of the STAT3 inhibitor curcubitacin to the culture also abrogated CXCL12-induced IL-10 production, confirming an important role for S727-STAT3 as a mediator of CXCL12-CXCR4-induced IL-10 production by CLL cells.

We next determined if activation of the CXCR4-CXCL12-STAT3-IL10 pathway in CLL is important in mediating their immunoregulatory function. Culture of primary CLL with CXCL12 induced significantly more suppression of CD3+ T cell function, including TNF-α, IFN-γ and IL-2 production, and CD107a degranulation, compared to CD3+ T cells cultured with untreated CLL cells or with CXCL12 alone. The addition of IL-10 blocking antibody to the co-culture completely reversed T cell dysfunction, supporting an important for IL-10 in CLL-mediated T-cell suppression. IL-10 has been reported to induce T cell suppression through phosphorylation of Y705-STAT3.^.Blocking IL-10 also prevented CLL-induced phosphorylation of Y705-STAT3 in T cells, confirming an important role for CLL-derived IL-10 in activation of Y705-STAT3 and induction of T cell dysfunction.

Lenalidomide is an immune-modulatory drug with clinical efficacy in CLL and was recently reported to inhibit STAT3 phosphorylation. To investigate if lenalidomide can inhibit CXCL12-induced STAT3 phosphorylation, we treated CLL cells with lenalidomide and measured p-S727-STAT3 levels. Exposure of CLL cells to 10µM/ml lenalidomide prevented CXCL12-induced increase
in p-S727-STAT3 and resulted in significant reduction in the IL-10 response by CLL cells. Lenalidomide also suppressed IL-10-induced Y705-STAT3 phosphorylation in healthy T cells, thus reversing CLL-induced T cell dysfunction.  We next confirmed the in vivo relevance of our findings using PBMC cryopreserved from patients treated with lenalidomide monotherapy (NCT00535873). When compared to pretreatment samples, CLL cells from on-treatment samples produced less IL-10 and showed significantly improved T cell function.

We thus conclude that the capacity of CLL to produce IL-10 is mediated by the CXCL12-CXCR4-STAT3 pathway and may contribute to immunodeficiency in patients. Lenalidomide can reverse CLL-induced immunosuppression through multiple mechanisms that involve abrogation of the CXCL12-CXCR4-S727STAT3-mediated IL-10 response by CLL B cells and prevention of IL-10-induced phosphorylation of Y705-STAT3 in T cells.