Correspondence: D MacGlashan, Johns Hopkins Asthma and Allergy Center, Johns Hopkins Medicine, Baltimore, MD 21224, USA. E-mail: dmacglas@jhmi.edu

Expression of high-affinity IgE receptors (FcRI) and histamine-containing granules are hallmarks of basophils, which highlights their similarity to mast cells. Historically, it is this similarity that has focused investigation into the biology of basophils and kept attention on its role in hypersensitivity reactions. Is it possible that there is more to the life of the basophil? Are they more than just terminally differentiated cannon fodder cells rapidly mobilized to repel ecto-parasites using their surface-bound IgE to bind parasite antigens that induce the release of potent mediators? Recent studies of murine basophils by Mack and co-workers published in Nature Immunology¹ have begun to suggest that the answer to these questions is yes (Figure 1).

Figure 1.

The basophil as a mediator of the immune response. Upper left: The classical primary immune response (CD4+ T cells, B cells, Ag-presenting cells (APC)) is initiated with the first presentation of Ag to the host. Following the production of IgG and IhE by plasma cells (PC), the basophil binds circulating Ab of either class. A second presentation with Ag results in Ag-loaded basophils that begin producing cytokines IL-4 and IL-6. The presentation of Ag and production of cytokines by basophils (upper right) enhances the secondary humoral immune response. The circumstances that differentiate this participatory role for basophils and its traditional ability to react like a mast cell with full degranulation, lipid mediator release and cytokine release to generate an anaphylactic reaction remain to be completely elucidated.

Full figure and legend (138K)

In some ways, the change in perspective about basophils began with the recognition that they secrete cytokines.^{2, 3} This followed on the heels of studies demonstrating that mast cell lines secreted cytokines⁴ but an interesting twist to these early studies was that basophils secreted IL-4 whereas mast cells did not. There remains some controversy about this point but basophils are easily demonstrated to produce large quantities of IL-4, and the functionally related cytokine and IL-13, whereas it has proven very difficult to coax mast cells of any source to produce measurable quantities of IL-4. The basophil became a missing link in how T-helper 2 (Th2) cells could be initially induced to differentiate down the Th2 pathway, a process requiring the presence of IL-4. A number of studies using human basophils and lymphocytes strongly suggested that the basophil was the major contributor of IL-4 early in inflammatory reactions. These observations led investigators to generate basophils in strains of mice, using knockout/knock-in approaches, that fluoresce green because green fluorescent protein (GFP) had been linked to the IL-4 promoter.^{5, 6}

Historically, the study of murine basophils has been controversial. Indeed, it was even proposed that they did not exist in mice. But as for most advances, tools that were lacking to readily identify murine basophils were eventually developed to the point that confidence in their presence was found. In some ways, studies of the IL-4 promoter linked GFP-positive cell in mouse bone marrow and blood provided the means to find other surface markers that uniquely identified the basophil and it is these tools that are in more common use today. However, it is worth stressing that the means to identify the mouse basophil remains a point of contention in various studies.

In early 2008, Sokol et al.⁷ provided fascinating evidence that murine basophils participated in immune deviation in a manner more direct than their generalized secretion of IL-4. Basophils could be found in the lymph nodes, juxtaposed to T cells, and IL-4 secretion could be initiated by a non-IgE-dependent stimulus associated with the loading/immunizing of mice with papain (the mechanism of this induction remains a mystery although it may be a direct effect of papain on basophils). In 2005, Mack et al.⁸ noted that a non-B, non-T antigen-binding cell that had long been observed in murine peripheral blood was a basophil. These studies led Mack et al.⁸ to explore whether this non-traditional antigen-binding cell has any function in normal immune responses, unrelated to their function in inflammatory reactions. Now, their recent study in Nature Immunology¹ suggests that basophils are important in modulating the secondary immune response that leads to augmented antibody production. Here, Denzel et al.¹ provide compelling evidence that basophils, through their immunoreceptor-dependent release of IL-6 and IL-4 and expression of CD40 L, assist CD4⁺ T cells in their regulation of the B-cell response to a second injection of immunizing antigen. The key tool in these studies was the development of a method to deplete the mouse of basophils using an anti-FcRI antibody. This is a critical point in interpretation of the results because in vitro studies demonstrate that this antibody strongly activates murine basophils. The studies do indicate, however, that the basophil is no longer detectable in bone marrow, blood or spleen and that there is no evidence for an increased level, in vivo, of IL-4 and IL-6 during or following treatment with anti-FcRI. However, this antibody also partially depleted mice of mast cells (about 50%). The study demonstrated that the same results occurred in mast cell-deficient W/W-v mice and the adoptive transfer of basophils, but not mast cells, provided the necessary augmentation of the secondary immune response.

The study shows that when basophils are depleted Ig levels are decreased following a secondary injection of antigen and the numbers of antigen-specific B cells are decreased. Basophils were shown to promote B-cell proliferation and modify the phenotype of T-helper cells, biasing them towards a 'B helper' phenotype with enhanced generation of IL-4, IL-10 and IL-13-producing CD4⁺ T cells. The basophil assistance was only effective during the secondary response because it was dependent on immunoglobulin receptors. Although IgE antibody played some role, much of the basophil activity persisted in FcRI knockout mice. Both IL-6 and IL-4 production by basophils appeared necessary although IL-4 was partially dispensable. Notably, after a primary immunization with pneumococcal surface protein A, the absence of basophils during a secondary challenge with a live S. pneumoniae resulted in a greater number of septic mice.

This study, together with that by Sokol et al.,⁷ lead to the conclusion that basophils have a function in general immune responses in mice. There may be some lingering concerns about the nature of the cell generating these behaviors or in the method used to deplete the mouse of basophils—the studies are critically dependent on what is happening during or after depletion. In addition, it is not clear from these studies whether basophils are induced to secrete more than the cytokines by the second immunization with the antigen. One might ask whether any form of standard hypersensitivity reaction (degranulation and lipid mediators) is involved, which is perhaps too mild to detect? In mice, basophil participation is often linked to IgG-mediated events rather than those mediated by IgE. Another example of this type of linkage is found in the studies of systemic anaphylaxis where basophils mediate a hypersensitive reaction that is also IgG-dependent.⁹ For those of us who have studied the human basophil, these studies are intriguing but some aspects are not immediately translatable. For example, human basophils cannot be activated through an IgG receptor. From the standpoint of the results by Denzel et al.,¹ where IL-6 production from basophils appears critical, human basophils have not been shown to produce IL-6 when activated through FcRI. Whether or not there will be human-specific ways of accomplishing the same activities—that is, T cell help, secondary immune response assistance—is yet to be determined. Unfortunately, the tools to answer the questions raised are not available for in vivo studies, so it may be some time before these observations are validated in humans. Nevertheless, the results raise interesting questions not just about the role of basophils but perhaps other granulocytes.