Calcium ion is a universal signaling intermediate, which is known to control various biological processes. In excitable cells, voltagegated calcium channels (Cav) are the major route of calcium entry and regulate multiple functions such as contraction, neurotransmitter release, and gene transcription. Here we show that T lymphocytes, which are nonexcitable cells, express both regulatory ␤ and pore-forming Cav1 ␣1 subunits of Cav channels, and we provide genetic evidence for a critical role of the Cav ␤3 and Cav ␤4 regulatory subunits in T lymphocyte function. Cav ␤-deficient T lymphocytes fail to acquire normal functions, and they display impairment in the T cell receptor-mediated calcium response, nuclear factor of activated T cells activation, and cytokine production. In addition, unlike in excitable cells, our data suggest a minimal physiological role for depolarization in Cav channel opening in T cells. T cell receptor stimulation induces only a small depolarization of T cells, and artificial depolarization of T cells using KCl does not lead to calcium entry. These observations suggest that the Cav channels expressed by T cells have adopted novel regulation͞gating mechanisms.C alcium ion plays critical and specific roles in various T cell functions, including activation, differentiation, proliferation, and cytokine production (1, 2). In T lymphocytes, ligation of the T cell receptor (TCR) by antigen leads to the release of calcium from intracellular stores, triggering the calcium release-activated calcium current (3), and a potential candidate for calcium release-activated calcium current channel was recently reported (4-6). But the complexity of the calcium response in T cells suggests the expression of more than one plasma membrane calcium channel. Although it is established that, in excitable cells, Cav channels constitute the major route of calcium entry (7), the functional presence of the Cav1 channels in T lymphocytes has been suggested (8-13) but has remained controversial because of the lack of a reliable and specific loss-of-function approach.Cav ␤ subunits are cytoplasmic proteins that strongly regulate Cav channels through direct interaction with pore-forming ␣1 subunits (14-17). The ␤ subunits are also critical for assembly of the channel complex (18), correct plasma membrane targeting (19,20), and stimulation of channel activity (21). A number of potential ␣1-␤ combinations are likely to form a Cav channel complex (22), and, among these, the ␤4 and ␤3 subunits are key subunits that associate with Cav1 channels (23-25).A spontaneous mutation named lethargic, which arose in the mouse inbred strain BALB͞cGn in 1962, is recognizable in homozygous mice at 2 weeks of age by the onset of ataxia, seizures, and lethargic behavior (26,27). These mice also exhibit a generalized immunological disorder including defective cell-mediated immune responses (28). It has been reported, using a positional cloning approach, that this syndrome was the result of a mutation of the Cav ␤4 subunit gene (29). This mutatio...

Engagement of the T cell antigen receptor (TCR) during antigen presentation initiates a coordinated action of a large number of signaling proteins and ion channels. AHNAK1 is a scaffold protein, highly expressed by CD4+ T cells, and is a critical component for calcium signaling. We showed that AHNAK1-deficient mice were highly susceptible to Leishmania major infection. AHNAK1-deficient CD4+ T cells responded poorly to TCR stimulation in vitro with low proliferation and low Interleukin-2 production. Furthermore, AHNAK1 deficiency resulted in a reduced calcium influx upon TCR crosslinking and subsequent poor activation of the transcription factor NFAT. AHNAK1 was required for plasma membrane expression of L-type calcium channels alpha 1S (Cav1.1), probably through its interaction with the beta regulatory subunit. Thus, AHNAK1 plays an essential role in T cell Ca2+ signaling through Cav1 channels, triggered via TCR activation; therefore, AHNAK1 is a potential target for therapeutic intervention.

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Immature B cells differentiate in the spleen into mature B cells, a process that is essential for their participation in the immune response. Previously, we showed that the MHC class II chaperone, invariant chain (Ii), controls this differentiation to the mature stage. Ii cytosolic domain-induced B cell maturation involves activation of transcription mediated by the NF-kappaB p65/RelA homodimer and requires the B cell enriched coactivator, TAF(II)105. In this study we show that the cytosolic region of Ii is cleaved within the plane of the membrane to generate a cytosolic fragment, which is essential for NF-kappaB activation and B cell differentiation. Our results suggest that Ii functions as a membrane-bound inactive inducer of NF-kappaB transcription that is activated by intramembrane proteolytic cleavage.

† Abdallah Badou and Mithilesh K. Jha have contributed equally to this work.In T lymphocytes, calcium ion controls a variety of biological processes including development, survival, proliferation, and effector functions. These distinct and specific roles are regulated by different calcium signals, which are generated by various plasma membrane calcium channels. The repertoire of calcium-conducting proteins in T lymphocytes includes store-operated CRAC channels, transient receptor potential channels, P2X channels, and L-type voltage-gated calcium (Ca v 1) channels. In this paper, we will focus mainly on the role of the Ca v 1 channels found expressed by T lymphocytes, where these channels appear to operate in aT cell receptor stimulation-dependent and voltage sensor independent manner. We will review their expression profile at various differentiation stages of CD4 and CD8 T lymphocytes. Then, we will present crucial genetic evidence in favor of a role of these Ca v 1 channels and related regulatory proteins in both CD4 and CD8T cell functions such as proliferation, survival, cytokine production, and cytolysis. Finally, we will provide evidence and speculate on how these voltage-gated channels might function in the T lymphocyte, a non-excitable cell.

Early stages of B cell development take place in the bone marrow, resulting in formation of immature B cells, which migrate to the spleen for their final differentiation into mature cells. This final maturation step is essential for B cells to become responsive to antigens and to participate in the immune response. Previously, we showed that the MHC class II chaperone, invariant chain (Ii), controls the differentiation of B cells from the immature to the mature stage. In this study, by generating transgenic mice expressing truncated Ii lacking its luminal domain, we could dissect the chaperonin activity of Ii from its role in B cell maturation. We demonstrate in vivo that Ii N-terminal domain is directly involved in the maturation of B cells and is sufficient to promote B cell differentiation.transgenic mice ͉ N-terminal domain ͉ immature B cell ͉ mature B cell ͉ antigen presentation B cell development involves the ordered progression of a stem cell through a number of stages, ultimately resulting in a mature B cell. There are various selective criteria that the cell must fulfill to complete this program. In the bone marrow, B cell development can be divided into different stages, based on the rearrangement status of the IgH and IgL chain loci (1, 2) and the expression of intracellular and surface-bound markers. This developmental program is controlled largely by a set of transcription factors and signaling pathways. The first cells expressing IgM at their surface during this developmental process are the immature B cells that leave the bone marrow and migrate to the spleen (3, 4). These immature B cells then penetrate the marginal zone sinus and reside in the outer zone of the periarteriolar lymphocytic sheath (5), where they become part of the B cell-rich follicular areas (5, 6). At this site in the spleen, B cells are still immature and can be distinguished from their mature counterparts (3,(7)(8)(9). The transition from immature to mature B cells is characterized by a series of changes in surface marker expression and in the activities of these cells. Recently, this transition stage was characterized further and divided into two populations. Transitional B cells of type 1 (T1) are the recent immigrants from the bone marrow. These cells develop into transitional B cells of type 2 (T2), which cycle and are found exclusively in the primary follicles of the spleen (10). Only 5-10% of the newly generated immature B cells are selected into the pool of long-lived, antigen-responsive mature B cells (9, 11). Why and how only such a small proportion of immature B cells is selected and by what molecular mechanism this selection occurs are largely unknown.Previously, we have shown that invariant chain (Ii), a MHC class II chaperone, plays a crucial role in B cell maturation (12). MHC class II molecules are heterodimeric complexes that present foreign antigenic peptides on the cell surface of antigenpresenting cells (APCs) to CD4 ϩ T cells (13)(14)(15). MHC class II synthesis and assembly begin in the endoplasmic reticul...

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Cytolytic CD8 ؉ T cells (CTLs) kill virally infected cells, tumor cells, or other potentially autoreactive T cells in a calcium-dependent manner. To date, the molecular mechanism that leads to calcium intake during CTL differentiation and function has remained unresolved. We demonstrate that desmoyokin (AHNAK1) is expressed in mature CTLs, but not in naive CD8 ؉ T cells, and is critical for calcium entry required for their proper function during immune response. We show that mature AHNAK1-deficient CTLs exhibit reduced Ca v1.1 ␣1 subunit expression (also referred to as L-type calcium channels or ␣1S pore-forming subunits), which recently were suggested to play a role in calcium entry into CD4 ؉ T cells. AHNAK1-deficient CTLs show marked reduction in granzyme-B production, cytolytic activity, and IFN-␥ secretion after T cell receptor stimulation. Our results demonstrate an AHNAK1-dependent mechanism controlling calcium entry during CTL effector function.