One question in the field was whether heterodimer formation was necessary for functional activity. production of cytokines that can directly and indirectly affect anti-tumor immune function and cancer cell growth. Signals that lead to the expansion of regulatory B cells and the spectrum of their functional roles are not well understood and are the subject of active research by many groups. Here, we elaborate broadly on the history of regulatory B cells in cancer and summarize recent studies that have established genetic models for the study BBD of regulatory B cell function and their potential for therapeutic intervention in the setting of solid cancers. Keywords: Regulatory B cells, cancer, IL-35, tumor immunology 1.?Introduction. This review highlights the recent studies on the role of regulatory B cells (Breg) in cancer. We provide a brief discussion of the role of regulatory immune BBD cells in development and disease, followed by a general summary of Breg cell phenotypes and function in solid malignancies. We discuss analysis of preclinical studies that demonstrate the detection methods, genetic models and functional studies on the role of IL-35 producing Breg cells in cancer growth and control of anti-tumor immune responses, and discuss the use of selective targeting of pathogenic B cell subsets. Finally, we highlight translational work describing phenotype, frequency, localization, and gene expression signature of Breg cells in patients with pancreatic cancer, that may enable studies of this B cell subset in many distinct cancer types. 2.?Regulatory function in homeostasis. The immune system serves to provide a potent barrier against invading pathogens. At the same time, there are mechanisms in place that ensure limited reactivity to potentially pathogenic self-antigens. Such protective measures are termed tolerance and encompass a range of mechanisms such as self-antigen-induced apoptosis and anergy, which are largely executed on a cell autonomous level. Extrinsic control of self-reactivity is perpetuated by a collection of peripheral immune cells that suppress unwanted immune cell activity via soluble mediators such as cytokines, direct cell-to-cell contact via membrane bound checkpoint mediators, or via induction of additional regulatory cell types (1, BBD 2). Several distinct regulatory cell types have been shown to contribute to tolerance against abundant self-antigens or those that may be temporarily overexpressed during key developmental processes. These include regulatory T cells (Treg), regulatory B cells (Breg), dendritic cells, macrophages, and myeloid derived suppressor cells (MDSC) among others (3, 4). Their tolerizing activity is shaped by maturation status, often accompanied by low expression of co-stimulatory machinery and microenvironmental cues (5, 6). For example, MDSCs are a heterogeneous population of immature myeloid cells which can suppress T cells, and in normal development are important in maintenance of immune tolerance at the maternalCfetal interface during pregnancy (7, 8). Myeloid cells also play an important role in tolerizing against the vast amounts of self-material that can be carried by apoptotic cells C this occurs via innate checkpoints such Tyro3, MDNCF Axl and MerTK receptor tyrosine kinases (9). Adaptive immune cell types also contribute to the maintenance of self-tolerance. Treg cells, characterized by expression of transcription factor studies evaluating IgG antibody responses in melanoma demonstrated effective killing of cancer cells by ADCC as well (64). Furthermore, in a mouse model of triple negative breast cancer, activation of T cells and antibody production by B cells impacted the response to checkpoint blockade immunotherapy (55). It is also important to note that in addition to producing antibodies, plasma cells secrete a myriad of cytokines that can recruit, activate, or suppress other immune cell populations (65). The role of plasma cells in the tumor microenvironment has been reviewed in detail in the excellent review by Sharonov et al. (36). 4.2. Antigen presentation. Although dendritic cells (DCs) are rightfully hailed as the masters of antigen presentation, DCs are often modified by tumors, hindering their capacity to present antigens. Therefore, the ability of B cells to present tumor-specific antigens to T cells may play a vital role in fostering anti-tumor immunity. For instance, dendritic cells harvested from cervical cancer patients were resistant to -CD40 treatment and incapable of driving an anti-tumor response, whereas B lymphocytes taken from the same patients responded to -CD40 treatment and consequently elicited secondary T cell responses (66). Moreover, antigen-presentation assays in a model of non-small-cell lung cancer supported the conclusion that activated B cells within the tumor were not only able to present antigen to CD4+ T cells, but also initiated an effector T cell phenotype (67). It should be noted, however, that exhausted tumor-infiltrating B cells in this same study led to the production of suppressive, regulatory BBD FoxP3+ CD4+ T cells (67). 4.3. Spatial organization. The recent evidence correlating the presence of B cells within.