T cell development ͉ T cell differentiation ͉ T cell signaling T he Tec family tyrosine kinase Itk is important for signaling downstream of the T cell receptor (1). In particular, Itkdeficient T cells have defects in phospholipase C-␥ (PLC-␥) phosphorylation, calcium mobilization, mitogen-activated protein kinase (MAP kinase) activation, and AP-1 and nuclear factor of activated T cells (NFAT) activation after T cell receptor (TCR) stimulation. Itk is also critical for conventional ␣ T cell development, selection, and function. Of particular importance, Itk signaling regulates CD4 ϩ T helper cell differentiation, playing a key role in the development of Th2 responses (2). Based on this welldocumented defect of Itk Ϫ/Ϫ mice in generating Th2 effector responses and cytokine production, it was surprising to discover that these mice had spontaneously elevated levels of serum IgE (3, 4), as B cell isotype switching to IgE is highly dependent on Th2 cytokines IL-4 and IL-13 (5). As our previous studies had indicated that Itk Ϫ/Ϫ ␣ TCR ϩ NKT cells (referred to as ␣ NKT cells) were also highly defective in producing effector cytokines such as IL-4 (6), we considered the possibility that ␥␦ TCR ϩ NKT cells were the major source of Th2 cytokines in Itk Ϫ/Ϫ mice.The ␥␦ T cells are a highly conserved subset of T cells that constitutes 1-5% of the lymphocytes in the blood and peripheral organs of mice but can account for up to 50% of the lymphocytes in the mucosal epithelia. As with other subsets of ''innate'' T cells, ␥␦ T cells express memory cell surface markers (7), and are capable of rapidly secreting effector cytokines (8). Among the many functions attributed to ␥␦ T cells, a great deal of recent interest has focused on their ability to modulate adaptive immune responses, specifically the humoral response (9).A variety of studies have indicated that ␥␦ T cells are able to provide help for B cell responses. Initial studies performed in mice lacking ␣ T cells showed that B cell expansion, differentiation, and secretion of 'T-dependent' antibody isotypes, IgE, and IgG 1 , were all intact in these mice (10). Furthermore, TCR Ϫ/Ϫ mice challenged repeatedly with parasitic infections could produce germinal centers and generate increased antibody production (11). Using a model of pulmonary allergic inflammation, decreased production of IgE and IgG 1 was seen in mice lacking ␥␦ T cells compared with WT mice (12). The ␥␦ T cells have also been shown to directly induce germinal center formation and Ig hypermutation (13). Interestingly, even though the ␥␦ T cells expressing CD4 account for only 5-10% of all ␥␦ cells, it is this subset that appears to be responsible for inducing germinal centers (14). Human ␥␦ T cells have also been found in germinal centers; these cells were found to up-regulate B cell costimulatory molecules such as CD40L, OX40, CD70, and inducible costimulatory molecule (ICOS) in response to TCR stimulation (15, 16). Together, these data indicate that ␥␦ T cells can promote, either directly or indirectly, th...