ICOS is critical for CD40-mediated antibody class switching.

The inducible co-stimulatory molecule (ICOS) is a CD28 homologue implicated in regulating T-cell differentiation. Because co-stimulatory signals are critical for regulating T-cell activation, an understanding of co-stimulatory signals may enable the design of rational therapies for immune-mediated diseases. According to the two-signal model for T-cell activation, T cells require an antigen-specific signal and a second, co-stimulatory, signal for optimal T-cell activation. The co-stimulatory signal promotes T-cell proliferation, lymphokine secretion and effector function. The B7-CD28 pathway provides essential signals for T-cell activation, but does not account for all co-stimulation. We have generated mice lacking ICOS (ICOS-/- ) to determine the essential functions of ICOS. Here we report that ICOS-/- mice exhibit profound deficits in immunoglobulin isotype class switching, accompanied by impaired germinal centre formation. Class switching was restored in ICOS-/- mice by CD40 stimulation, showing that ICOS promotes T-cell/B-cell collaboration through the CD40/CD40L pathway.

CTLA-4 regulates induction of anergy in vivo.

The requirement for CTLA-4 during the induction of peripheral T cell tolerance in vivo was investigated using naive TCR transgenic T cells lacking CTLA-4. CTLA-4(-/-) T cells are resistant to tolerance induction, as demonstrated by their proliferative responses, IL-2 production, and progression into the cell cycle. Following exposure to a tolerogenic stimulus in vivo and restimulation in vitro, wild-type T cells are blocked at the late G1 to S restriction point of the cell cycle. In contrast, CTLA-4(-/-) T cells enter into the S phase of the cell cycle, as shown by downregulation of p27(kip1), elevated cdk2 kinase activity, and Rb hyperphosphorylation. Thus, CTLA-4 has an essential role in determining the outcome of T cell encounter with a tolerogenic stimulus.

The development of CD4+ T effector cells during the type 2 immune response.

Multiple pathways may be involved in the development of interleukin 4 (IL-4) producing T helper (Th) cells and the associated type 2 immune response. Increasing evidence suggests that the strength of signals delivered to the T cell may favor the development of the type 2 response. In contrast, antigen-presenting cell- (APC) derived stimuli produced following pattern recognition receptor binding during the innate response promotes the development of interferon-gamma (IFN-gamma) producing cells and the associated type 1 immune response. In many cases, the balance between increased signaling strength and the innate response may determine whether the type 2 response develops. T cell receptor (TCR), CD4, and costimulatory molecule interactions may all contribute to signal strength, but the type 2 immune response may be particularly dependent on the availability of coreceptor and costimulatory molecule interactions. B7 ligand interactions are required for the development of the type 2 immune response and interaction of CD28 with either B7-1 or B7-2 can provide sufficient signals for its initiation. In B7-2-deficient mice, the initial type 2 immune response is intact, but the response is not sustained, suggesting that B7-2 is important at later stages of the type 2 immune response. The roles of CD28 and CTLA-4 during the type 2 response remain unclear. The type 2 response to infectious pathogens is pronounced in CD28-/- mice, suggesting that other costimulatory molecule interactions can substitute for CD28 for the development of IL-4 producing T cells and the associated type 2 immune response.

CTLA-4 and T cell activation.

The past year has seen significant advances in our understanding of the role of cytotoxic T lymphocyte antigen 4 (CTLA-4) in regulating T cell activation and tolerance. Recent studies indicate that CTLA-4 not only counterbalances CD28 signals but also can inhibit T cell responses independently of CD28. Recent work has also revealed a role for CTLA-4 in regulating Th1/Th2 differentiation. Manipulation of CTLA-4 in animal models of autoimmunity has shown that CTLA-4 regulates both the initiation and the progression of autoimmune diseases.

B7-2 is required for the progression but not the initiation of the type 2 immune response to a gastrointestinal nematode parasite.

T cells require CD28/CTLA-4 costimulatory molecule interactions in addition to Ag-specific signals through the TCR for in vivo effector Th cell function. Some studies have suggested that the ligands for these costimulatory molecules may differentially influence effector T cell function with B7-2 favoring a type 2 response and B7-1 favoring a type 1 response, while other studies have suggested that these molecules may be redundant. The recent development of B7-2-deficient mice permits the direct analysis of the requirement of B7-2 during a type 2 immune response to an infectious pathogen. We have examined, in B7-2-deficient mice, effector Th cell function and the associated type 2 immune response following infection with Heligmosomoides polygyrus, a natural murine parasitic nematode. Elevations in cytokine gene expression and protein secretion were pronounced and comparable in inoculated B7-2-/- and B7-2+/+ mice at day 8 after H. polygyrus inoculation. However, by day 14 after infection, increases in T cell cytokine expression were markedly inhibited in H. polygyrus-inoculated B7-2-/- mice. Furthermore, elevations in serum IgE and germinal center formation were inhibited at later stages of the immune response, while elevations in serum IgG1 persisted. These findings suggest that certain T-dependent components vary in their B7-2-dependency during the type 2 immune response. They further demonstrate that B7-2 interactions are not necessary for the initiation of the type 2 immune response, but are instead required for its progression after the development of effector T cells.

Do effector and memory T helper cells also need B7 ligand costimulatory signals?

Blocking B7 ligand-costimulatory molecules can inhibit a primary T-dependent immune response, but whether these interactions also mediate ongoing or memory immune responses is less clear. Development of immunotherapies based on blocking B7 ligand interactions would be limited if they were effective only at the initiation of an immune response. We discuss the conditions under which T helper effector and memory cells may or may not require B7 ligand interactions for their function.

CD28 dependence of T cell differentiation to IL-4 production varies with the particular type 2 immune response.

T cell differentiation to effector cell function is required for the development of a type 2 immune response. The T cell surface molecule, CD28, is widely considered to be the principal costimulatory molecule involved in T cell differentiation to effector function, including IL-4 production, although this has been difficult to directly examine in vivo. We have studied in vivo differentiation to T cell effector function during two type 2 immune responses in CD28 knockout mice: the systemic immune response to goat anti-mouse IgD Ab and the mucosal immune response following oral inoculation with the nematode parasite, Heligmosomoides polygyrus. Our results show that in C57BL/6 CD28 knockout mice elevations in IL-4 gene expression and protein secretion are blocked during the immune response to goat anti-mouse IgD, and associated increases in serum IgG1 and IgE are also inhibited to untreated control levels. In marked contrast, T cell differentiation to IL-4 production is comparable in C57BL/6 CD28 -/- and CD28 +/+ H. polygyrus-inoculated mice, and elevations in both serum IgG1 and IgE levels occur. These results indicate that the specific kind of type 2 immune response determines whether T cell differentiation to IL-4 production is CD28 dependent.

Effects of blocking B7-1 and B7-2 interactions during a type 2 in vivo immune response.

The costimulatory signal provided to T cells through CD28/CTLA-4 interactions is required for in vivo Th cell effector function associated with cytokine production. However, it is uncertain whether the two well-characterized ligands for these molecules, B7-1 and B7-2, differentially influence the consequent development of a type 1 or a type 2 primary response. We have examined the in vivo effects of blocking B7-1 and/or B7-2 ligand interactions on the type 2 mucosal immune response that follows oral infection of mice with the nematode parasite, Heligmosomoides polygyrus. Administration of the combination of anti-B7-1 and anti-B7-2 Abs inhibited H. polygyrus-induced increases in serum IgG1 and IgE levels, the expansion of mesenteric lymph node (MLN) germinal centers, in situ CD4+ T cell expansion, elevated blood eosinophils, and increased intestinal mucosal mast cells. Similarly, both Abs blocked MLN and Peyer's patch cytokine gene expression and elevations in MLN T cell-derived IL-4 protein secretion. However, in the same experiments, administration of either anti-B7-1 or anti-B7-2 Abs alone had little effect on any of these parameters. T cell and B cell activation was also blocked by the combination of anti-B7-2 and a B7-1-specific mutant Y100F CTLA-4Ig construct. These results suggest that to the extent that anti-B7-1 and anti-B7-2 mAbs block B7 interactions, either B7-1 or B7-2 ligand interactions can provide the required costimulatory signals that lead to T cell effector function during a type 2 in vivo immune response.