Differential regulatory effects of intercellular adhesion molecule-1 on costimulation by the CD28 counter-receptor B7.

Although ligation of the CD3/TCR complex initiates an activation signal in T cells, additional costimulatory signals generated during cell-to-cell interactions with APC transduced via ligation of CD11a/CD18 and CD28 by their specific counter-receptor intercellular adhesion molecule (ICAM)-1 and B7, respectively, are required for optimal T cell proliferation and cytokine synthesis. Using soluble IgC gamma 1 fusion proteins of these costimulatory counter-receptors, we have recently shown that unactivated resting CD4+ T cells and Ag-primed CD4+ T cells differ in their response to the costimulation by ICAM-1 and B7. Preferential proliferative responses of resting T and Ag-primed T cells to ICAM-1 and B7, respectively, prompted us to speculate that ICAM-1-induced signals may regulate coupling of the CD28 signaling pathway. Furthermore, both B7 and ICAM-1 are co-expressed on APC and thus, may co-regulate activation-driven maturation of T cells. In this study, we have examined regulatory effects of IgC gamma 1 fusion proteins of B7, ICAM-1, and ICAM-2 (a homologue of ICAM-1) on each other's costimulation. We first demonstrate that TCR-directed costimulation of resting CD4+ T cells with ICAM-1 (ICAM-1 priming) but not ICAM-2 induces increased responsiveness to B7. Priming of CD4+ T cells with ICAM-1 induced higher expression of both CD18 and CD28 than that with either B7 or ICAM-2. Cross-linking of CD28 induced faster and significantly higher cytoplasmic free calcium mobilization response in ICAM-1-primed CD4+ T cells than in resting, B7-primed, or ICAM-2-primed CD4+ T cells. B7 synergized with ICAM-1 but not ICAM-2 to augment proliferative responses of not only resting CD4+ T cells but also those that had been primed with either ICAM. Unlike resting or ICAM-2-primed CD4+ T cells, ICAM-1-primed CD4+ T cells efficiently proliferated in response to the synergistic costimulation of B7 and ICAM-2. In contrast, both ICAM-1 and ICAM-2 inhibit B7-driven proliferation of Ag-primed CD4+ T cells. Thus, B7 and ICAM-1 exert contrasting regulatory effects on the proliferation of CD4+ T cells depending on their state of activation-induced maturation.     

A recombinant vector expressing transgenes for four T-cell costimulatory molecules (OX40L,B7-1, ICAM-1, LFA-3) induces sustained CD4+ and CD8+ T-cell activation,protection from apoptosis,and enhanced cytokine production

The role of OX40L on the activation of T cells was investigated using poxvirus vectors expressing OX40L alone or in combination with three other T-cell costimulatory molecules: B7-1, ICAM-1, and LFA-3. Poxvirus vector-infected cells were used to stimulate nai;ve or activated CD4(+) and CD8(+) T cells. These studies demonstrate that (a) OX40L plays a role in sustaining the long-term proliferation of CD8(+) T cells in addition to the known effect on CD4(+) T cells following activation, (b) OX40L enhances the production of Th1 cytokines (IL-2, IFN-gamma, and TNF-alpha) from both CD4(+) and CD8(+) while no change in IL-4 expression was observed, and (c) the anti-apoptotic effect of OX40L on T cells is likely the result of sustained expression of anti-apoptotic genes while genes involved in apoptosis are inhibited. In addition, these are the first studies to demonstrate that the combined use of a vector driving the expression of OX40L with three other costimulatory molecules (B7-1, ICAM-1, and LFA-3) both enhances initial activation and then further potentiates sustained activation of nai;ve and effector T cells.     

Intercellular adhesion molecule 1 is critical for activation of CD28-deficient T cells

Presentation of Ag to T lymphocytes in the absence of the requisite costimulatory signals leads to an Ag-specific unresponsiveness termed anergy, whereas Ag presentation in conjunction with costimulation leads to clonal expansion. B7/CD28 signaling has been shown to provide this critical costimulatory signal and blockade of this pathway may inhibit in vitro and in vivo immune responses. Although T cells from CD28-deficient mice are lacking in a variety of responses, they nonetheless are capable of various primary and secondary responses without the induction of anergy expected in the absence of costimulation. This suggests that there may be alternative costimulatory pathways that can replace CD28 signaling under certain circumstances. In this paper, we show that ICAM-1becomes a dominant costimulatory molecule for CD28-deficient T cells. ICAM-1 costimulates anti-CD3-mediated T cell proliferation and IL-2 secretion in CD28-deficient murine T cells. Furthermore, splenocytes from ICAM-1-deficient mice could not activate CD28-deficient T cells and splenocytes lacking both ICAM and CD28 fail to proliferate in response to anti-CD3-induced T cell signals. This confirms that not only can ICAM-1 act as a CD28-independent costimulator, but it is the dominant, requisite costimulatory molecule for the activation of T cells in the absence of B7/CD28 costimulation.     

The cytoplasmic domain of CD28 is both necessary and sufficient for costimulation of interleukin-2 secretion and association with phosphatidylinositol 3''-kinase.

T-cell activation requires two signaling events. One is provided by the engagement of the T-cell antigen receptor, and the second represents a costimulatory signal provided by antigen-presenting cells. CD28 mediates a costimulatory signal by binding its ligands, B7-1 and B7-2, on antigen-presenting cells, but the signaling pathway activated by CD28 has not been identified. A homologous molecule, CTLA-4, expressed on activated T cells, also binds to B7-1 and B7-2, but whether it has a signaling function is not known. We performed a structure-function analysis of CD28 to identify the functional domain which activates signal transduction. Truncation of the 40-amino-acid CD28 cytoplasmic tail abrogated costimulatory signaling. Chimeric constructs containing the extracellular and transmembrane regions of CD8 linked to the cytoplasmic region of CD28 had a costimulatory signaling function. Similar chimeras containing the cytoplasmic tail of CTLA-4 did not signal. Thus, the cytoplasmic region of CD28, but not CTLA-4, is sufficient to mediate costimulatory signaling. In addition, after CD28 stimulation, the p85 subunit of phosphatidylinositol 3'-kinase and phosphatidylinositol 3'-kinase activity were found in CD28 immunoprecipitates. The CD8-CD28 chimera, which has a costimulatory signaling function, associates with p85, while the nonfunctioning CD8-CTLA-4 chimera and a CD8-zeta chimera do not associate with p85. These results suggest that phosphatidylinositol 3'-kinase is specifically activated by CD28 and may mediate proximal events in the costimulatory signaling pathway regulated by CD28.     

CD80 (B7-1) and CD86 (B7-2) are functionally equivalent in the initiation and maintenance of CD4+ T-cell proliferation after activation with suboptimal doses of PHA

Effective activation of T cells requires engagement of two separate T-cell receptors. The antigen-specific T-cell receptor (TCR) binds foreign peptide antigen-MHC complexes, and the CD28 receptor binds to the B7 (CD80/CD86) costimulatory molecules expressed on the surface of antigen-presenting cells (APC). The simultaneous triggering of these T-cell surface receptors with their specific ligands results in an activation of this cell. In contrast, CTLA-4 (CD152) is a distinct T-cell receptor that, upon binding to B7 molecules, sends an inhibitory signal to T cell activation. Many in vitro and in vivo studies demonstrated that both CD80 and CD86 ligands have an identical role in the activation of T cells. Recently, functions of B7 costimulatory molecules in vivo have been investigated in B7-1 and/or B7-2 knockout mice, and the authors concluded that CD86 could be more important for initiating T-cell responses, while CD80 could be more significant for maintaining these immune responses. In this study, we directly compared the role of CD80 and CD86 in initiating and maintaining proliferation of resting CD4(+) T cells in an in vitro mode system that allowed to provide the first signal-to-effector cells through the use of suboptimal doses of PHA and the second costimulatory signal through cells expressing CD80 or CD86, but not any other costimulatory molecules. Using this experimental system we demonstrate that the CD80 and CD86 molecules can substitute for each other in the initial activation of resting CD4(+) T cells and in the maintenance of their proliferative response.     

Characterization of endogenous Chinese hamster ovary cell surface molecules that mediate T cell costimulation

Chinese hamster ovary (CHO) cells are commonly used in the generation of transfectants for use in in vitro costimulation assays. However, we have noted that nontransfected CHO cells can themselves provide a low-level B7/CD28 independent costimulatory signal for CD3-mediated murine T cell activation and IL-2 production. This study set out to identify those molecules that contribute to this CHO-dependent costimulatory activity. We describe a CHO subline capable of delivering potent CD28-independent costimulation to murine T cells and the generation of monoclonal antibodies against these CHO cells that inhibited this costimulatory activity. These blocking antibodies do not affect CHO cell-independent costimulation or bind mouse cells, suggesting an effect mediated by their target molecules on the costimulatory competent CHO cells. Immunoprecipitation and expression cloning revealed that these antibodies bound the hamster homologues of Crry (CD21/35), CD44, CD54 (ICAM-1), CD63, CD87, CD147, and an 80- to 90-kDa protein which could not be cloned. Expression of these hamster genes on COS cells demonstrated that hamster CD54 was able to costimulate both CD3-mediated IL-2 secretion and T cell proliferation by naive murine T cells independent of the other molecules identified.     

Intercellular adhesion molecule-2, a second counter-receptor for CD11a/CD18 (leukocyte function-associated antigen-1), provides a costimulatory signal for T-cell receptor-initiated activation of human T cells.

Activation of T cells often requires both activation signals delivered by ligation of the TCR and those resulting from costimulatory interactions between certain T cell surface accessory molecules and their respective counter-receptors on APC. CD11a/CD18 complex on T cells modulate the activation of T cells by interacting with its counter-receptors intracellular adhesion molecule (ICAM-1) (CD54) and/or ICAM-2 on the surface of APC. The costimulatory ability of ICAM-1 has been demonstrated. Using a soluble ICAM-2 Ig fusion protein (receptor globulin, Rg) we demonstrate the costimulatory effect of ICAM-2 during the activation of CD4+ T cells. When coimmobilized with anti-TCR-1 mAb ICAM-2 Rg induced vigorous proliferative response of CD4+ T cells. This costimulatory effect of ICAM-2 was dependent on its coimmobilization with mAb directed at the CD3/TCR complex but not those directed at CD2 or CD28. Both resting as well as Ag-primed CD4+ T cells responded to the costimulatory effects of ICAM-2. The addition of mAb directed at the CD11a or CD18 molecules almost completely inhibited the responses to ICAM-2 Rg. These results are consistent with the role of CD11a/CD18 complex as a receptor for ICAM-2 mediating its costimulatory effects. Stimulation of T cells with coimmobilized anti-TCR-1 and ICAM-2 resulted in the induction of IL-2R (CD25), and anti-Tac (CD25) mAb inhibited this response suggesting the contribution of endogenously synthesized IL-2 during this stimulation. These results demonstrate that like its homologue ICAM-1, ICAM-2 also exerts a strong costimulatory effect during the TCR-initiated activation of T cells. The costimulatory effects generated by the CD11a/CD18:ICAM-2 interaction may be critical during the initiation of T cell activation by ICAM-1low APC.     

Differential costimulatory effects of adhesion molecules B7, ICAM-1, LFA-3, and VCAM-1 on resting and antigen-primed CD4+ T lymphocytes.

Optimal proliferation of T cells although initiated via ligation of the CD3/TCR complex requires additional stimulation resulting from adhesive interactions between costimulatory receptors (R) on T cells and their counter-R on APC. At least four distinct adhesion molecules (counter-R) present on APC, B7, ICAM-1 (CD54), LFA-3 (CD58), and VCAM-1 have been individually shown to costimulate T cell activation. Because some of these molecules may be expressed simultaneously on APC, it has been difficult to examine relative contributions of individual counter-R during the induction of T cell proliferation. We have produced soluble IgC gamma 1 fusion chimeras (receptor globulins or Rg) of B7, ICAM-1, LFA-3, and VCAM-1 and compared their relative abilities to costimulate proliferation of resting or Ag-primed CD4+ T cells. When co-immobilized with mAb directed at TCR alpha beta or CD3 but not CD2 or CD28, each Rg induced proliferation of both resting and Ag-primed CD4+ cells. In contrast, similarly co-immobilized CD7 Rg or ELAM-1 Rg were ineffective. Resting CD4+ T cells produced more IL-2, expressed significantly higher levels of IL-2R alpha, and proliferated more efficiently when costimulated with either ICAM-1 Rg or VCAM-1 Rg than with B7 Rg or LFA-3 Rg. CD4+ CD45RO+ memory T cells proliferated more vigorously in response to the costimulation by each of the four Rg than CD4+ CD45RA+ naive T cells. In contrast with the behavior of resting CD4+ T cells, proliferation of Ag-preactivated CD4+ T cells was most efficient when costimulated by B7 Rg. The costimulatory effect of LFA-3 Rg on Ag-primed CD4+ T cells was weaker than that of B7 Rg but was significantly greater than that of either ICAM-1 Rg or VCAM-1 Rg. These results suggest that resting and Ag-primed CD4+ T cells preferentially respond by proliferation to different costimulatory counter-R. ICAM-1 and VCAM-1 may be involved in the initiation of proliferation of Ag-responsive T cells, and B7 and LFA-3 may facilitate sustained proliferation of Ag-primed T cells. The cumulative costimulation by the above counter-R may facilitate optimal expression of various regulatory and effector functions of T cells.     

CD28-mediated costimulation in the absence of phosphatidylinositol 3-kinase association and activation.

T-cell activation involves two distinct signal transduction pathways. Antigen-specific signaling events are initiated by T-cell receptor recognition of cognate peptide presented by major histocompatibility complex molecules. Costimulatory signals, which are required for optimal T-cell activation and for overcoming the induction of anergy, can be provided by the homodimeric T-cell glycoprotein CD28 through its interaction with the counterreceptors B7-1 and B7-2 on antigen-presenting cells. Ligation of CD28 results in its phosphorylation on tyrosines and the subsequent recruitment and activation of phosphatidylinositol 3-kinase (PI 3-kinase). It has been suggested that the induced association of CD28 and PI 3-kinase is required for costimulation. We report here that ligation of CD19, a heterologous B-cell receptor that also associates with and activates PI 3-kinase upon ligation, failed to costimulate interleukin-2 production. Moreover, pharmacological inhibition of PI 3-kinase activity failed to block costimulation mediated by CD28. By mutational analysis, we demonstrate that disruption of PI 3-kinase association with CD28 also did not abrogate costimulation. These results argue that PI 3-kinase association with CD28 is neither necessary nor sufficient for costimulation of interleukin-2 production. Finally, we identify specific amino acid residues required for CD28-mediated costimulatory activity.     

A novel bispecific tetravalent antibody fusion protein to target costimulatory activity for T-cell activation to tumor cells overexpressing ErbB2/HER2

Persistent activation of T-lymphocytes requires two signals: one is initiated by T-cell receptor binding to antigenic peptide presented by MHC molecules. In addition, binding of the B7 family members CD80 or CD86 on professional antigen presenting cells to CD28 on T cells is considered to provide an important costimulatory signal. Activation without costimulation induces T-cell unresponsiveness or anergy. To selectively localize costimulatory activity to the surface of tumor cells and enhance activation of tumor-specific T cells, we have developed a novel molecular design for bispecific costimulatory proteins with antibody-like structure. Within a single polypeptide chain we have assembled the IgV-like, CD28-binding domain of human CD86 (CD86(111)) together with hinge, CH2 and CH3 domains of human IgG1, and the scFv(FRP5) antibody fragment which recognizes the ErbB2 (HER2) protooncogene present at high levels on the surface of many human tumor cells. Upon expression in the yeast Pichia pastoris, the resulting CD86(111)-IgG-scFv(FRP5) protein could be purified as a homodimeric, tetravalent molecule from culture supernatants using single-step affinity chromatography. Bispecific binding of the molecule to ErbB2 on the surface of tumor cells and to the B7 counter receptor CTLA-4 was demonstrated by FACS analysis. Potent costimulatory activity of chimeric CD86(111)-IgG-scFv(FRP5) was confirmed by its ability to stimulate the proliferation of primary human lymphocytes pre-activated by low concentrations of anti-CD3 antibody. Our results suggest that such multivalent soluble proteins which combine specific targeting to tumor cells with costimulatory activity may become useful tools to elicit and/or improve T-cell mediated, tumor-specific immune responses.     

Evaluation of the T cells and costimulatory molecules in the protective efficacy of 30 kDa secretory protein against experimental tuberculosis

Phenotypic changes of T lymphocytes and B7 costimulatory molecules in mice first vaccinated with mycobacterial 30 kDa secretory protein and then challenged with Mycobacterium tuberculosis H37Rv (Group 2) were monitored using flow cytometry and compared with non-vaccinated, but challenged mice (Group 1). In Group 1, the proportion of CD3+ and CD4+ T cells increased until 28 days postinfection (p.i.) and then declined to levels even less than healthy controls (non-vaccinated and non-challenged healthy mice), especially at later stages of infection (i.e. 72 days p.i.). However, the levels of CD8+ T cells did not decline and remained either significantly higher or similar to healthy control levels. In Group 2, however, the levels of CD3+ and CD4+ T cells did not decline as seen in Group 1, but remained significantly higher than in Group 1. Furthermore, the profile of CD8+ T cells remained similar to what was observed in Group 2. In order to elucidate Th1-Th2 bias, the ratio of IgG2a/IgG1 bearing cells was enumerated by flow cytometry. A predominantly Th1 response was observed in Group 1 until 28 days p.i. (IgG2a/IgG1 ratio was >1). However, in Group 2 a predominantly Th1 bias was observed throughout the period studied in terms of IgG2a/IgG1 ratios. The examination of expression of B7-1 and B7-2 costimulatory molecules on a monocyte gated population was carried out. In Group 2, the B7-1 and B7-2 expression was found to be significantly higher compared to Group 1, especially at later stages of infection (i.e. 60 and 72 days p.i.). Thus, these results suggest the capability of mycobacterial 30 kDa secretory protein in restoring the T-cell responses, especially at later stages of infection, possibly by augmentation of both B7-1 and B7-2. Further, these costimulatory molecules are probably required for effective T-cell responses against virulent mycobacterial challenge in a murine model of tuberculosis.     

Human acute lymphoblastic leukemia (ALL) blasts as accessory cells during T-cell activation: differences between patients in costimulatory capacity affect proliferative responsiveness and cytokine release by activated T cells

The ability of acute lymphoblastic leukemia (ALL) blasts to mediate costimulatory signals during T-lymphocyte activation was investigated in an experimental model in which monoclonal T-cell populations were stimulated with standardized activation signals (anti-CD3 and anti-CD28 monoclonal antibodies; phytohemagglutinin, PHA). Leukemia cells from 12 consecutive ALL patients with high peripheral blood blast counts were studied. Proliferative T-cell responses were detected for a majority of these patients when irradiated leukemia blasts were used as accessory cells during activation. T-cell cytokine release was also observed for most patients when using nonirradiated ALL accessory cells. Low or undetectable cytokine levels were usually observed for CD8+ clones, whereas the CD4+ clones often showed a broad cytokine response with release of interleukin-2 (IL-2), IL-4, IL-10, IL-13 and interferon gamma(IFN-gamma) in the presence of the ALL accessory cells. ALL blasts were also able to function as allostimulatory cells for normal peripheral blood mononuclear responder cells. However, both T-cell proliferation and cytokine release showed a wide variation between ALL patients. The accessory cell function of ALL blasts showed no correlation with the release of immunomodulatory mediators (IL-2, IL-10, IL-15) or the expression of any single adhesion/costimulatory membrane molecule (CD54, CD58, CD80, CD86) by the blasts. We conclude that for a majority of patients, native ALL blasts can mediate costimulatory signals needed for accessory cell-dependent T-cell activation, but differences in costimulatory capacity between ALL patients affects both the proliferative responsiveness and cytokine release by activated T cells.     

Expression and contribution of B7-1 (CD80) and B7-2 (CD86) in the early immune response to Leishmania major infection.

CD28 interactions promote T cell responses, and whether B7-1 or B7-2 is utilized may influence Th cell subset development. CD28 blockade by CTLA-4Ig treatment or by targeted gene disruption has yielded different conclusions regarding the role of CD28 in the development of Th1 and Th2 cells following Leishmania major infection. In this study, we demonstrate that B7-mediated costimulation is required for the development of the early immune response following infection of resistant or susceptible mice. In contrast, CD28-/- BALB/c mice infected with L. major produce cytokines comparable to those of infected wild-type mice. Treatment of CD28-/- mice with CTLA-4Ig did not diminish this response, suggesting that a B7-independent pathway(s) contributes to the early immune response in these mice. In conventional BALB/c or C3H mice, B7-2 functions as the dominant costimulatory molecule in the initiation of early T cell activation following L. major infection, leading to IL-4 or IFN-gamma production, respectively. The preferential interaction of B7-2 with its ligand(s) in the induction of these responses correlates with its constitutive expression relative to that of B7-1. However, B7-1 can equally mediate costimulation for the production of either IL-4 or IFN-gamma when expressed at high levels. Thus, in leishmaniasis, costimulation involving B7-1 or B7-2 can result in the production of either Th1 or Th2 cytokines, rather than a preferential induction of one type of response.     

LICOS, a primordial costimulatory ligand?

In mammals, the classical B7 molecules expressed on antigen-presenting cells, B7-1 (CD80) and B7-2 (CD86), bind the structurally related glycoproteins CD28 and CTLA-4 (CD152), generating costimulatory signals that regulate the activation state of T cells. A recently identified human CD28-like protein, ICOS, also induces costimulatory signals in T cells when crosslinked with antibodies, but it is unclear whether ICOS is part of a B7-mediated regulatory pathway of previously unsuspected complexity, or whether it functions independently and in parallel. Here, we report that, rather than binding B7-1 or B7-2, ICOS binds a new B7-related molecule of previously unknown function that we call LICOS (for ligand of ICOS). At 37 degrees C, LICOS binds only to ICOS but, at lower, non-physiological temperatures, it also binds weakly to CD28 and CTLA-4. Sequence comparisons suggest that LICOS is the homologue of a molecule expressed by avian macrophages and of a murine protein whose expression is induced in non-lymphoid organs by tumour necrosis factor alpha (TNFalpha). Our results define the components of a distinct and novel costimulatory pathway and raise the possibility that LICOS, rather than B7-1 or B7-2, is the contemporary homologue of a primordial vertebrate costimulatory ligand.     

Resting B lymphocytes as APC for naive T lymphocytes: dependence on CD40 ligand/CD40

Although resting B cells as APC are tolerogenic for naive T cells in vivo, we show here that they can provide all the costimulatory signals necessary for naive T cell proliferation in vivo and in vitro. In the absence of an activating signal through the B cell Ag receptor, T cell proliferation after Ag recognition on resting B cells depends on CD40 expression on the B cells, implying that naive T cells use the membrane-bound cytokine, CD40 ligand (CD154), to induce the costimulatory signals that they need. Induction of B7-1 (CD80) and increased or sustained expression of CD44H, ICAM-1 (CD54), and B7-2 (CD86) are dependent on the interaction of CD40 ligand with CD40. Transient expression (12 h) of B7-2 is T cell- and peptide Ag-dependent, but CD40-independent. Only sustained (>/=24 h) expression of B7-2 and perhaps increased expression of ICAM-1 could be shown to be functionally important in this system. T cells cultured with CD40-deficient B cells and peptide remain about as responsive as fresh naive cells upon secondary culture with whole splenic APC. Therefore, B cells, and perhaps other APC, may be tolerogenic not because they fail to provide sufficient costimulation for T cell proliferation, but because they are deficient in some later functions necessary for a productive T cell response.     

A role for inducible costimulator protein in the CD28- independent mechanism of resistance to Toxoplasma gondii

Long-term resistance to Toxoplasma gondii is dependent on the development of parasite-specific T cells that produce IFN-gamma. CD28 is a costimulatory molecule important for optimal activation of T cells, but CD28(-/-) mice are resistant to T. gondii, demonstrating that CD28-independent mechanisms regulate T cell responses during toxoplasmosis. The identification of the B7-related protein 1/inducible costimulator protein (ICOS) pathway and its ability to regulate the production of IFN-gamma suggested that this pathway may be involved in the CD28-independent activation of T cells required for resistance to T. gondii. In support of this hypothesis, infection of wild-type or CD28(-/-) mice with T. gondii resulted in the increased expression of ICOS by activated CD4(+) and CD8(+) T cells. In addition, both costimulatory pathways contributed to the in vitro production of IFN-gamma by parasite-specific T cells and when both pathways were blocked, there was an additive effect that resulted in almost complete inhibition of IFN-gamma production. Although in vivo blockade of the ICOS costimulatory pathway did not result in the early mortality of wild-type mice infected with T. gondii, it did lead to increased susceptibility of CD28(-/-) mice to T. gondi associated with reduced serum levels of IFN-gamma, increased parasite burden, and increased mortality compared with the control group. Together, these results identify a critical role for ICOS in the protective Th1-type response required for resistance to T. gondii and suggest that ICOS and CD28 are parallel costimulatory pathways, either of which is sufficient to mediate resistance to this intracellular pathogen.     

Direct activation of innate and antigen-presenting functions of microglia following infection with Theiler's virus

Microglia are resident central nervous system (CNS) macrophages. Theiler's murine encephalomyelitis virus (TMEV) infection of SJL/J mice causes persistent infection of CNS microglia, leading to the development of a chronic-progressive CD4(+) T-cell-mediated autoimmune demyelinating disease. We asked if TMEV infection of microglia activates their innate immune functions and/or activates their ability to serve as antigen-presenting cells for activation of T-cell responses to virus and endogenous myelin epitopes. The results indicate that microglia lines can be persistently infected with TMEV and that infection significantly upregulates the expression of cytokines involved in innate immunity (tumor necrosis factor alpha, interleukin-6 [IL-6], IL-18, and, most importantly, type I interferons) along with upregulation of major histocompatibility complex class II, IL-12, and various costimulatory molecules (B7-1, B7-2, CD40, and ICAM-1). Most significantly, TMEV-infected microglia were able to efficiently process and present both endogenous virus epitopes and exogenous myelin epitopes to inflammatory CD4(+) Th1 cells. Thus, TMEV infection of microglia activates these cells to initiate an innate immune response which may lead to the activation of naive and memory virus- and myelin-specific adaptive immune responses within the CNS.     

Suppressive effects of CTLA4-Ig on nasal allergic reactions in presensitized murine model

Ag-specific T cell activation requires the engagement of T cell receptor (TCR) with antigen in the context of MHC, and the engagement of appropriate costimulatory molecules. It is well established that B7/CD28-CTLA4 costimulatory pathway plays an important role in the induction of T helper (Th) cells in T-cell dependent immune reactions. In this study, we evaluated the effects of blocking the costimulatory pathway by systemic administration of CTLA4-Ig during repeated nasal antigen challenges in systemically presensitized mouse. The antigen-induced early phase nasal symptoms, nasal hyperresponsiveness to histamine and nasal eosinophilia were significantly suppressed by CTLA4-Ig treatment. Elevation of serum level of antigen-specific IgE, but not IgG1 or IgG2a was inhibited by the treatment. In relation to cytokine levels in the tissue extracts of the nasal mucosa, an up-regulation of IL-4 was significantly inhibited, however, the levels of IL-5 and IFN-gamma were not affected by the treatment. These results suggest that B7/CD28-CTLA4 costimulatory pathway plays an important role in on-going Th2-related allergic reactions in the nose.     

CD28 signaling augments Elk-1-dependent transcription at the c-fos gene during antigen stimulation.

Untransformed CD4(+) Th1 cells stimulated with Ag and APC demonstrated a dependence on B7- and CD28-mediated costimulatory signals for the expression and function of AP-1 proteins. The induction of transactivation by the c-fos gene regulator Elk-1 mirrored this requirement for TCR and CD28 signal integration. c-Jun N-terminal kinase (JNK) (but not extracellular signal-regulated kinase or p38) protein kinase activity was similarly inhibited by neutralizing anti-B7 mAbs. Blockade of JNK protein kinase activity with SB 202190 prevented both Elk-1 transactivation and c-Fos induction. These results identify a unique role for B7 costimulatory molecules and CD28 in the activation of JNK during Ag stimulation in Th1 cells, and suggest that JNK regulates Elk-1 transactivation at the c-fos gene to promote the formation of AP-1 complexes important to IL-2 gene expression.