Wong, Tina W.

Abstract Abstract 2917 Multiple myeloma (MM) is a cancer in the bone marrow (BM) characterized by the accumulation of transformed plasma cells (PCs). The pre-malignant form of the disease, monoclonal gammopathy of undetermined significance (MGUS), shares many of the genetic abnormalities found in MM, including chromosomal translocations, hyperdiploidy, and gene-specific mutations. Given this, we believe other factors within the tumor microenvironment must contribute to disease progression by influencing cell survival and/or proliferation. Eosinophils (Eos) are granulocytic leukocytes that are best known for their involvement in host immune defense and pathologic states such as allergy and asthma. Recently, they were also shown to play a role in the regulation of murine BM PC homeostasis via their secretion of IL-6 and APRIL. Murine BM PCs and Eos both express CXCR4 and are believed to home to CXCL12 expressing stromal cells (SCs) in the BM. The goal of this study, therefore, was to investigate whether Eos, or soluble mediators released by Eos, have biological activity on MM cells. Thus, it is possible that this type of innate immune cell may be present in the tumor microenvironment in MM patients to support disease progression. To our knowledge, a potential role for Eos in MM has not been previously studied. We began our studies by assessing whether Eos are co-localized with normal BM PCs and/or MM PCs. Immunofluorescence analysis of BM core biopsies from normal subjects revealed occasional colocalization of PCs with Eos. Similarly, MM biopsies showed regions of MM cell clusters with increased Eos density, suggesting possible biological interactions. However, we also observed regions of MM cell clusters that lack Eos, which could indicate the liberation of these transformed cells from the requirement of Eos for survival/proliferation. Next, using Eos isolated from human BM aspirates and a panel of disease-relevant human MM cell lines (HMCLs) extensively characterized in our laboratory, we aimed to verify that MM cells and Eos could both migrate toward the chemokine CXCL12. Our data showed that indeed the KAS-6/1, ANBL-6, DP-6, and KP-6 HMCLs and human BM Eos all migrated toward this chemokine. We then determined if Eos had biological activity towards MM cells as revealed by enhanced DNA synthesis. Consistent with our interpretation of the immunofluorescence stained sections, the proliferation of some, but not all, HMCLs was enhanced when cocultured with Eos isolated from either human BM or peripheral blood. To address whether contact between Eos and HMCLs was required for this phenomenon, we assessed HMCL proliferation upon treatment with Eos culture supernatant (SN). Our data suggest that the effect of Eos on the Eos-inducible HMCLs can be contact-independent as treatment of these HMCLs with Eos SN increased their proliferation. Similarly, proliferation of primary CD138+ MM cells was also enhanced when treated with Eos SN. Because BM PCs reside in niches that include support cells such as SCs and that proliferation of MM PCs is enhanced in the presence of SCs, we next questioned whether Eos can substitute for SCs in this niche or if Eos and SCs support PC survival/proliferation through different mechanisms. We observed that Eos and BM SCs together stimulated more HMCL proliferation than either cell type did alone, indicating the presence of non-redundant roles for the two cell types. Finally, we began to investigate the mechanism by which Eos enhance HMCL proliferation. In contrast to prior reports that murine Eos express IL-6, mRNA transcripts of IL-6, a known proliferation-inducing cytokine for MM cells, were not detected in human Eos. Moreover, analysis of the Eos SN showed an absence of IL-6. Neutralization of IL-6 in the HMCL-Eos coculture did not abolish the induced proliferation. Altogether, these data suggest that human Eos utilize a yet to be identified, IL-6-independent mechanism to support malignant PC proliferation. Taken together, our data show that Eos and MM cells can colocalize in the BM via their migration toward a common source of CXCL12, i.e. BM SCs, to create a niche that promotes tumor cell growth. Eos can enhance malignant PC proliferation via soluble products, although additional contact-dependent effects may exist as well and have not been explored. Additional studies are currently underway to further characterize the mechanism by which Eos may influence the biology of MM in the tumor microenvironment. Disclosures: No relevant conflicts of interest to declare.

Abstract The view of eosinophils (Eos) as solely effector cells involved in host parasite defense and in the pathophysiology of allergic diseases has been challenged in recent years. In fact, there is a growing realization that these cells interact with other components of innate and adaptive immunity. For example, mouse Eos were recently demonstrated to promote plasma cell retention in the bone marrow. However, it remains unknown whether Eos influence the biology of normal B lymphocytes. In this study, we specifically assessed the effect of Eos on B cell survival, proliferation, and Ig secretion. Our data first revealed that the genetic deletion of Eos from NJ1638 IL-5 transgenic hypereosinophilic mice (previously shown to display profound B cell expansion) resulted in the near abolishment of the B cell lymphocytosis. In vitro studies using human tissues demonstrated Eos’ proximity to B cell follicles and their ability to promote B cell survival, proliferation, and Ig secretion via a contact-independent mechanism. Additionally, this ability of Eos to enhance B cell responsiveness was observed in both T-independent and T-dependent B cell activation and appears to be independent of the activation state of Eos. Finally, a retrospective clinical study of hypereosinophilic patients revealed a direct correlation between peripheral blood eosinophil levels and B cell numbers. Taken together, our study identifies a novel role for Eos in the regulation of humoral immunity via their impact on B cell homeostasis and proliferation upon activation.