Several environmental factors can differentially regulate monocyte and macrophage response patterns, resulting in the display of distinct functional phenotypes. Galectin-1, an endogenous lectin found at peripheral lymphoid organs and inflammatory sites, has shown immunoregulatory activity in vivo in experimental models of autoimmunity and cancer. Whereas compelling evidence has been accumulated regarding the effects of galectin-1 on T cell fate, limited information is available on how galectin-1 may impact other immune cell types. In the present study, we report a novel role for galectin-1 in the regulation of monocyte and macrophage physiology. Treatment with galectin-1 in vitro differentially regulates constitutive and inducible FcγRI expression on human monocytes and FcγRI-dependent phagocytosis. In addition, galectin-1 inhibits IFN-γ-induced MHC class II (MHC-II) expression and MHC-II-dependent Ag presentation in a dose-dependent manner. These regulatory effects were also evident in mouse macrophages recruited in response to inflammatory stimuli following treatment with recombinant galectin-1 and further confirmed in galectin-1-deficient mice. Investigation of the mechanisms involved in these functions showed that galectin-1 does not affect survival of human monocytes, but rather influences FcγRI- and MHC-II-dependent functions through active mechanisms involving modulation of an ERK1/2-dependent pathway. Our results provide evidence of a novel unrecognized role for galectin-1 in the control of monocyte/macrophage physiology with potential implications at the crossroad of innate and adaptive immunity.

Kaposi’s sarcoma (KS), a multifocal vascular neoplasm linked to human herpesvirus-8 (HHV-8/KS-associated herpesvirus [KSHV]) infection, is the most common AIDS-associated malignancy. Clinical management of KS has proven to be challenging because of its prevalence in immunosuppressed patients and its unique vascular and inflammatory nature that is sustained by viral and host-derived paracrine-acting factors primarily released under hypoxic conditions. We show that interactions between the regulatory lectin galectin-1 (Gal-1) and specific target N-glycans link tumor hypoxia to neovascularization as part of the pathogenesis of KS. Expression of Gal-1 is found to be a hallmark of human KS but not other vascular pathologies and is directly induced by both KSHV and hypoxia. Interestingly, hypoxia induced Gal-1 through mechanisms that are independent of hypoxia-inducible factor (HIF) 1α and HIF-2α but involved reactive oxygen species–dependent activation of the transcription factor nuclear factor κB. Targeted disruption of Gal-1–N-glycan interactions eliminated hypoxia-driven angiogenesis and suppressed tumorigenesis in vivo. Therapeutic administration of a Gal-1–specific neutralizing mAb attenuated abnormal angiogenesis and promoted tumor regression in mice bearing established KS tumors. Given the active search for HIF-independent mechanisms that serve to couple tumor hypoxia to pathological angiogenesis, our findings provide novel opportunities not only for treating KS patients but also for understanding and managing a variety of solid tumors.