Purpose Human tumors consist of heterogeneous populations of cells with distinct marker expression and functional properties. In squamous cell carcinoma of the head and neck (SCCHN), CD44 is a well-characterized marker of a resilient subpopulation of cells associated with increased tumorigenesis, radioresistance, and chemoresistance. Evidence indicates that these cells have an immune suppressive phenotype; however, mechanisms have been elusive. Experimental Design Using primary human SCCHN tumor samples and patient-derived xenografts, we examined the phenotypes of subsets of tumor cells and investigated mechanisms regulating their immunogenicity. Results CD44+ cells in primary human SCCHN were found to have an epithelial-to-mesenchymal (EMT) phenotype and were less immunogenic than CD44− cells when cultured with autologous CD8+ tumor-infiltrating T cells. Selective expression of the programmed death-ligand 1 (PD-L1) was observed on CD44+ cells compared to CD44− cells and was associated with constitutive phosphorylation of STAT3 on CD44+ cells. Importantly, inhibition of STAT3 decreased expression of PD-L1 on CD44+ cells. Interferon-γ (IFNγ) treatment preferentially induced even further PD-L1 expression on CD44+ cells and was associated with enhanced IFNγ receptor expression and phosphorylation of STAT1. Finally, the decreased immunogenicity of CD44+ cells was partially reversed by antibody blockade of the programmed death 1 (PD-1) receptor, indicating that the differences in PD-L1 expression between CD44+ and CD44− cells are biologically and clinically relevant. Conclusions Our findings provide a mechanism by which long-lived CD44+ tumor-initiating cells can selectively evade host immune responses and provide rationale for targeting the PD-1 pathway in the adjuvant therapy setting of SCCHN.

Myelomonocytic cells play a key role in the progression of many solid tumors. However, very little is known about their contribution to the progression of hematopoietic cancers. We investigated the role of monocytes in the progression of human B-cell precursor acute lymphoblastic leukemia (BCP-ALL). We demonstrated that coculturing human monocytes in vitro with CD19 ؉ BCP-ALL blasts from patients "conditioned" them to an inflammatory phenotype characterized by significant up-regulation of the chemokine, CXCL10. This phenotype was also observable ex vivo in monocytes isolated from BCP-ALL patients, which show elevated CXCL10 production compared with monocytes from healthy donors. Functionally, the "conditioned" monocytes promoted migration and invasive capacity of BCP-ALL cells. Increased invasion was mediated by matrix metalloproteinase 9 expression and activity in the BCP-ALL cells induced by the monocyte-derived CXCL10. However, neither the "conditioned" monocytes nor the CXCL10 produced by these cells had any effect on the proliferation/viability of BCP-ALL cells and angiogenesis. Collectively, our results strongly suggest a protumoral role for human monocytes in BCP-ALL, orchestrated by CXCL10 and its effect on tumor cell migration and invasion. These observations highlight the importance of the CXCL10/CXCR3 chemokine circuit in BCP-ALL progression. (Blood. 2012;119(1): 227-237) IntroductionMonocytes/macrophages are versatile innate immune cells that play a key role in host defense by performing a wide variety of functions. 1,2 Depending on the microenvironmental cue, monocytes/ macrophages can display distinct phenotypes and functions. 1,3,4 For example, microbial stimuli (eg, lipopolysaccharide) and IFN-␥ induce these cells to an M1 or "classical" activation state, which is characterized by up-regulation of inflammatory cytokines (eg, IL-12, IL-23, and TNF) and potent microbicidal as well as tumoricidal properties. In contrast, IL-4 and IL-13 induce an M2 (or "alternative") activation state, which is characterized by high expression of anti-inflammatory cytokines (eg, IL-10), scavenging receptors (eg, mannose receptor, CD163), and efficient phagocytic ability. These cells facilitate the resolution of inflammation, tissue repair or remodeling, and tumor-promoting activities. 5 However, it may be pointed out that M1 and M2 phenotypes represent 2 extremes of a spectrum of macrophage functional states, 3,4 and cells with overlapping M1-M2 characteristics have been noted under certain pathophysiologic settings. 6,7 In recent years, several studies have focused on the contribution of myelomonocytic cells in cancer progression. 3,8 Monocytes/ macrophages are recruited to tumor tissues where they promote tumor growth, proliferation, angiogenesis, and metastasis through the release of factors, such as TNF, IL-6, VEGF, IL-8, and matrix metalloproteinases (MMPs). 7,9,10 In line with this fact, a proangiogenic population of monocytes (termed as Tie-2-expressing monocytes) has been recently identified as the principal ...