Basophils: new players in the cytokine network.

Basophils belong to a myeloid cell population that has been ignored for more than a century, mainly because of its paucity, its lack of specific markers, and the absence of experimental models. Given that in mice, even the mere existence of basophils was contested, they were alluded to as "histamine-producing cells" or "non-T non-B cells" in initial studies. It is now widely acknowledged that basophils respond to various IgE-dependent or -independent stimuli, and are engaged in a complex cross talk with a number of immunocompetent cells (T or B lymphocytes, macrophages, dendritic cells, endothelial cells…). Indeed, on the one hand they are critically involved during the onset, the effector phase and exacerbation of T(H)2 responses through their capacity to generate large amounts of cytokines with pro-T(H)2 functions (IL-4, IL-13 TSLP, IL-25), on the other hand, they contribute to immunoglobulin synthesis and class switching, angiogenesis, autoimmunity, tumor immunity and hematopoiesis by producing cytokines such as IL-6, VEGF, GM-CSF and IL-3. Finally, it has been established that they can present antigens to CD4+ or CD8+ T cells in an MHC class II- or class I-dependent manner, respectively. Taken together, these activities confer important immunoregulatory functions upon basophils, both in innate and adaptive immunity.