Targeting Dysfunctional Myeloid Cells Delays Disease Development and Improves Immune Function in a CLL Mouse Model

Chronic lymphocytic leukemia (CLL) is a malignancy of mature B cells that is characterized by apoptosis resistance and dysfunctional immune system. The chronic nature and slow development of the disease indicates a contribution of CLL-induced inflammation in the disease course. Previous reports suggested a potential role of myeloid cells in mediating these defects. However, the composition and function of CLL-associated myeloid cells have not been thoroughly investigated in an in vivo system. Here, we used the well-established CLL mouse model, Eµ-TCL1 mice (TCL1), to characterize changes within myeloid cell populations along with CLL development and the influence of their depletion on disease progression and immune dysfunction.

We have recently shown that CLL development in TCL1 mice is associated with massive changes within myeloid cell populations. In the peritoneal cavity (PC) of leukemic mice we observed an infiltration of monocytes and an M2-like skewing of macrophages according to phenotypical and signaling signatures. Along this line, monocytes infiltrated the spleens of leukemic animals, both in primary CLL and adoptive transfer models, which is most likely due to high CCL2 serum levels. These monocytes lost the inflammatory Ly6C^hi subset and were severely skewed towards Ly6C^low patrolling monocytes, accompanied by high expression of adhesion and angiogenic molecules like ICAM1, PECAM1 and MMP14. Gene expression profiling of splenic myeloid cells from TCL1 mice revealed an enrichment of various genes involved in dendritic cell (DC) maturation and MHC-II-mediated antigen presentation. However, the numbers of MHC-II^hi mature DCs and macrophages were significantly decreased, suggesting a monocyte differentiation arrest leading to impaired anti-tumor immune response. The observed transcriptional upregulation of multiple inflammatory cytokines like TNF-α, CXCL9, CXCL10 and CXCL16 in monocytes was confirmed by serum cytokine arrays, and is likely due to the overexpression of the pro-inflammatory regulator TREM-1. In addition, TCL1 monocytes upregulated the expression of several inhibitory molecules like PD-L1, IL-10, IL1ra and IL4i1 suggesting an impaired immune function.

While CLL-induced immune dysfunction is a well-established phenomenon, the contribution of myeloid cells in this context was not clear. We therefore sought to determine the in vivo effects of myeloid cell depletion on CLL development and its associated immune defects. For that purpose we used liposomal clodronate to selectively ablate macrophages and monocytes from young wild-type mice adoptively transferred with murine CLL. Our data clearly show control of CLL development in clodronate-treated mice relative to control liposomes as demonstrated by decreased spleen weight (1.09 vs. 0.54 g, p < 0.0001) and a significant drop in tumor load, defined as CD5⁺CD19⁺ cells, in spleen (60.58% vs. 42.25%), peripheral blood (43% vs 11.8%), PC (66.2% vs 3.1%), lymph nodes (4.9% vs 1.2%) and bone marrow (1.9% vs 0.8%). In addition, we observed changes in immune effector cells in response to myeloid cell depletion suggesting better immune status in treated mice. Interestingly, the loss of macrophages/monocytes was compensated by increased splenic monocyte proliferation as shown by EdU incorporation in vivo. In contrast to control mice, the repopulating monocytes upon clodronate treatment were largely inflammatory Ly6C^hi monocytes.

In summary, our data show that skewing of myeloid cells actively contributes to CLL development via; 1) enhancing the survival of leukemic cells, and 2) suppressing anti-tumor immune functions. In the absence of monocytes and macrophages, disease development is delayed in mice adoptively transferred with murine CLL. Therefore, we suggest that targeting non-malignant myeloid cells in CLL might serve as a novel strategy for CLL immunotherapy.