Inducible costimulator is required for type 2 antibody isotype switching but not T helper cell type 2 responses in chronic nematode infection

The role of ICOS in allergic disease: Positive or Negative?

With the rapid increase in the incidence of allergic diseases, the mechanisms underlying the development of these diseases have received a great deal of attention, and this is particularly true in regard to the role of ICOS in allergic diseases. Current studies have revealed that ICOS affects the functional activity of multiple immune cells that modulate the adaptive immune system. Additionally, ICOS also plays a crucial role in mediating cellular immunity and coordinating the response of the entire immune system, and thus, it plays a role in allergic reactions. However, the ICOS/ICOS-ligand (ICOS-L) axis functions in a dual role during the development of multiple allergic diseases. In this review, we explore the role of ICOS/ICOSL in the context of different immune cells that function in allergic diseases, and we summarize recent advances in their contribution to these diseases.

Switched phenotypes of macrophages during the different stages of Schistosoma japonicum infection influenced the subsequent trends of immune responses

BACKGROUND

Macrophages play crucial roles in immune responses during the course of schistosomal infections.

METHODS

We currently investigated influence of immunocompetent changes in macrophages via microarray-based analysis, mRNA expression analysis, detection of serum cytokines, and subsequent evaluation of the immune phenotypes following the differentiation of infection-induced lymphocytes in a unique T1/T2 double-transgenic mouse model.

RESULTS

The gradual upregulation of genes encoding YM1, YM2, and interleukin (IL)-4/IL-13 receptors in infected mice indicated the role of type 2 alternatively activated macrophages (M2, AAMφs) in immune responses after Schistosoma japonicum egg production. FACS analysis showed that surface markers MHC class II (IA/IE) and CD8α⁺ of the macrophages also exhibited a dramatic change at the various time points before and after egg-production. The transgenic mouse experiments further demonstrated that the shifting of macrophage phenotypes influenced the percentage of helper T (Th)-2 cells, which was observed to be higher than that of Th1 cells, which increased only at 3 and 5 weeks post-infection. The differentiation of effector B cells showed a similar but more significant trend toward type-2 immunity.

CONCLUSION

These results suggest that the infection of mice with S. japonicum resulted in a final Th2- and Be2-skewed immune response. This may be due to phenotypic changes in the macrophages. The influence of alternatively activated macrophages was also activated by S. japonicum egg production. This study elucidated the existence of variations in immune mechanisms at the schistosome infection stages.

Spontaneous Differentiation of T Follicular Helper Cells in LATY136F Mutant Mice

Mice with a mutation at the LAT-PLCγ1 binding site (Y136) have a defect in thymocyte development due to dampened TCR signaling. CD4⁺ T cells that do reach the periphery are hyper-activated and skewed to Th2. Over time, these mice develop an autoimmune-like syndrome, characterize by overproduction of Th2 cytokines, T cell infiltration into various organs, and B cell activation, isotype switching, and autoantibody production. In this study, we examined IL4 production by CD4⁺ T cells in the LATY136F mice using the KN2 reporter mice, in which human CD2 expression marks T cells that are actively producing IL4 protein. We showed that these mice had spontaneous Tfh differentiation. Despite the fact that the majority of CD4⁺ T cells were skewed to Th2 and were GATA3⁺, only a small subset of them were actively secreting IL4. These T cells were Tfh cells that expressed BCL6 and were localized to B cell-rich germinal centers within the spleen. Interestingly, these Tfh cells expressed high levels of both BCL6 and GATA3. By using LAT conditional knockout mice that inducibly express only the LATY136F allele, we further showed that Tfh cell differentiation was likely the result of defective LAT-PLCγ1 signaling in the periphery. In addition, B cells were required for spontaneous development of Tfh cells and uncontrolled T cell expansion in these mice. Together, these results indicated a novel role for tonic LAT-PLCγ1 signaling in modulating Tfh cell differentiation during development of autoimmune syndrome.

ICOS Co-Stimulation: Friend or Foe?

Over the last 15 years, the inducible T cell co-stimulator (ICOS) has been implicated in various immune outcomes, including the induction and regulation of Th1, Th2, and Th17 immunity. In addition to its role in directing effector T cell differentiation, ICOS has also been consistently linked with the induction of thymus-dependent (TD) antibody (Ab) responses and the germinal center (GC) reaction. ICOS co-stimulation, therefore, appears to play a complex role in dictating the course of adaptive immunity. In this article, we summarize the initial characterization of ICOS and its relationship with the related co-stimulatory molecule CD28. We then address the contribution of ICOS in directing an effector T cell response, and ultimately disease outcome, against various bacterial, viral, and parasitic infections. Next, we assess ICOS in the context of TD Ab responses, connecting ICOS signaling to follicular helper T cell differentiation and its role in the GC reaction. Finally, we address the link between ICOS and human autoimmune disorders and evaluate potential therapies aiming to mitigate disease progression by modulating ICOS signaling.

Follicular helper T cells mediate IgE antibody response to airborne allergens

BACKGROUND

T_H2 cells have long been believed to play a pivotal role in allergic immune responses, including IgE antibody production and type 2 cytokine-mediated inflammation and pathology. A new T-cell subset, follicular helper T (T_FH) cells, is specialized in supporting B-cell maturation and antibody production.

OBJECTIVE

We sought to investigate the roles of T_FH cells in allergic immune responses.

METHODS

Naive mice were exposed to cytokines or natural allergens through the airways. Development of allergic immune responses was analyzed by collecting draining lymph nodes and sera and by challenging the animals. Cytokine reporter mice and gene-deficient mice were used to dissect the immunologic mechanisms.

RESULTS

We observed the development of IL-4-producing T_FH cells and T_H2 cells in draining lymph nodes after airway exposure to IL-1 family cytokines or natural allergens. T_FH and T_H2 cells demonstrated unique phenotypes, tissue localization, and cytokine responses. T_FH cells supported the sustained production of IgE antibody in vivo in the absence of other T-cell subsets or even when T_H2 cell functions were severely compromised. Conversely, conditional deficiency of the master regulator Bcl6 in CD4⁺ T cells resulted in a marked reduction in T_FH cell numbers and IgE antibody levels, but type 2 cytokine responses and eosinophilic inflammation in the airways remained unaffected.

CONCLUSION

T_FH cells play critical roles in the regulation of IgE antibody production. Allergic immune responses to airborne allergens likely involve 2 distinct subsets of IL-4-producing CD4⁺ T cells, namely T_FH and Th2 cells.

The Costimulatory Molecule ICOS Regulates Host Th1 and Follicular Th Cell Differentiation in Response to Plasmodium chabaudi chabaudi AS Infection

Blood-stage Plasmodium chabaudi chabaudi AS infection requires cell- and Ab-mediated immunity to control acute and persistent infection, respectively. ICOS regulates CD4(+) T cell activation and promotes the induction of follicular Th (TFH) cells, CD4(+) T cells that support B cell affinity maturation within germinal centers (GCs), resulting in the production of high-affinity Abs. In this article, we demonstrate that, in response to P. c. chabaudi AS infection, the absence of ICOS resulted in an enhanced Th1 immune response that reduced peak parasitemia. Despite the absence of ICOS, CD4(+) T cells were capable of expressing PD-1, B cell lymphoma 6, and CXCR5 during early infection, indicating TFH development was not impaired. However, by day 21 postinfection, Icos(-/-) mice accumulated fewer splenic TFHs compared with Icos(+/+) mice, leading to substantially fewer GC B cells and a decrease in affinity, but not production, of parasite-specific isotype-switched Abs. Moreover, treatment of mice with anti-ICOS ligand Abs to modulate ICOS-ICOS ligand signaling revealed a requirement for ICOS in TFH differentiation only after day 6 postinfection. Ultimately, the quality and quantity of isotype-switched Abs produced in Icos(-/-) mice declined over time, resulting in impaired control of persistent parasitemia. Collectively, these data suggest ICOS is not required for TFH induction during P. c. chabaudi AS infection or production of isotype-switched Abs, but it is necessary for maintenance of a sustained high-affinity, protective Ab response.

The differential expression of IL-4 and IL-13 and its impact on type-2 immunity

Allergic disease represents a significant global health burden, and disease incidence continues to rise in urban areas of the world. As such, a better understanding of the basic immune mechanisms underlying disease pathology are key to developing therapeutic interventions to both prevent disease onset as well as to ameliorate disease morbidity in those individuals already suffering from a disorder linked to type-2 inflammation. Two factors central to type-2 immunity are interleukin (IL)-4 and IL-13, which have been linked to virtually all major hallmarks associated with type-2 inflammation. Therefore, IL-4 and IL-13 and their regulatory pathways represent ideal targets to suppress disease. Despite sharing many common regulatory pathways and receptors, these cytokines perform very distinct functions during a type-2 immune response. This review summarizes the literature surrounding the function and expression of IL-4 and IL-13 in CD4+ T cells and innate immune cells. It highlights recent findings in vivo regarding the differential expression and non-canonical regulation of IL-4 and IL-13 in various immune cells, which likely play important and underappreciated roles in type-2 immunity.

ICOS controls Foxp3(+) regulatory T-cell expansion, maintenance and IL-10 production during helminth infection

Foxp3⁺ regulatory T (Treg) cells are key immune regulators during helminth infections, and identifying the mechanisms governing their induction is of principal importance for the design of treatments for helminth infections, allergies and autoimmunity. Little is yet known regarding the co-stimulatory environment that favours the development of Foxp3⁺ Treg-cell responses during helminth infections. As recent evidence implicates the co-stimulatory receptor ICOS in defining Foxp3⁺ Treg-cell functions, we investigated the role of ICOS in helminth-induced Foxp3⁺ Treg-cell responses. Infection of ICOS^−/− mice with Heligmosomoides polygyrus or Schistosoma mansoni led to a reduced expansion and maintenance of Foxp3⁺ Treg cells. Moreover, during H. polygyrus infection, ICOS deficiency resulted in increased Foxp3⁺ Treg-cell apoptosis, a Foxp3⁺ Treg-cell specific impairment in IL-10 production, and a failure to mount putatively adaptive Helios⁻Foxp3⁺ Treg-cell responses within the intestinal lamina propria. Impaired lamina propria Foxp3⁺ Treg-cell responses were associated with increased production of IL-4 and IL-13 by CD4⁺ T cells, demonstrating that ICOS dominantly downregulates Type 2 responses at the infection site, sharply contrasting with its Type 2-promoting effects within lymphoid tissue. Thus, ICOS regulates Type 2 immunity in a tissue-specific manner, and plays a key role in driving Foxp3⁺ Treg-cell expansion and function during helminth infections.

Retnla (relmalpha/fizz1) suppresses helminth-induced Th2-type immunity

Retnla (Resistin-like molecule alpha/FIZZ1) is induced during Th2 cytokine immune responses. However, the role of Retnla in Th2-type immunity is unknown. Here, using Retnla^−/− mice and three distinct helminth models, we show that Retnla functions as a negative regulator of Th2 responses. Pulmonary granuloma formation induced by the eggs of the helminth parasite Schistosoma mansoni is dependent on IL-4 and IL-13 and associated with marked increases in Retnla expression. We found that both primary and secondary pulmonary granuloma formation were exacerbated in the absence of Retlna. The number of granuloma-associated eosinophils and serum IgE titers were also enhanced. Moreover, when chronically infected with S. mansoni cercariae, Retnla^−/− mice displayed significant increases in granulomatous inflammation in the liver and the development of fibrosis and progression to hepatosplenic disease was markedly augmented. Finally, Retnla^−/− mice infected with the gastrointestinal (GI) parasite Nippostrongylus brasiliensis had intensified lung pathology to migrating larvae, reduced fecundity, and accelerated expulsion of adult worms from the intestine, suggesting Th2 immunity was enhanced. When their immune responses were compared, helminth infected Retnla^−/− mice developed stronger Th2 responses, which could be reversed by exogenous rRelmα treatment. Studies with several cytokine knockout mice showed that expression of Retnla was dependent on IL-4 and IL-13 and inhibited by IFN-γ, while tissue localization and cell isolation experiments indicated that eosinophils and epithelial cells were the primary producers of Retnla in the liver and lung, respectively. Thus, the Th2-inducible gene Retnla suppresses resistance to GI nematode infection, pulmonary granulomatous inflammation, and fibrosis by negatively regulating Th2-dependent responses.

Retnla is a member of a family of cysteine-rich secreted proteins, referred to as ‘resistin-like molecules’ or ‘found in inflammatory zone’ that increase in expression during allergic reactions and following infection with a variety of metazoan parasites. Retnla was originally hypothesized to function as an effector molecule during helminth-induced Th2-type immune responses. Studies conducted here with Retnla-deficient mice, however, suggest that Retnla primarily functions as a regulatory molecule during helminth infection. Using three helminth model systems affecting three different organ systems, we show that Retlna is induced by IL-4 and IL-13 as a mechanism to suppress Th2-type immunity. Retnla deficiency increased inflammation in the lung following i.v. challenge with Schistosoma mansoni eggs. Retnla deficiency also accelerated the development of liver fibrosis following S. mansoni infection. This finding was particularly surprising since Retnla was previously shown to activate collagen-producing fibroblasts that induce fibrosis. Thus, Retnla may represent a novel target for the treatment of fibrotic diseases. Finally, resistance to the intestinal nematode parasite Nippostrongylus brasiliensis was significantly increased in the absence of Retnla. When viewed together, the combined results from all three models establish a critical role for the Th2-inducible gene Retnla (Fizz1/Relm-alpha) in the suppression of helminth-induced Th2-type immunity.

Cytokine-secreting follicular T cells shape the antibody repertoire

High-affinity antibodies are critical for host protection and underlie successful vaccines. The generation of such antibodies requires T cell-dependent help, which mediates germinal center reactions in which mutation and selection of B cells occurs. Using an interleukin 4-reporter system, we show here that CD4(+) follicular helper T cells constituted essentially all of the cytokine-secreting T cells in lymph nodes and were functionally distinct from T cells secreting the same cytokine in peripheral tissues. Follicular helper T cells with different cytokine profiles could be isolated as conjugates with B cells undergoing cytokine-specific immunoglobulin class switching with evidence of somatic hypermutation. Our findings support a model in which B cells compete for cytokines produced by follicular helper T cells that shape the affinity and isotype of the antibody response.

ICOS costimulation expands Th2 immunity by augmenting migration of lymphocytes to draining lymph nodes

The T cell costimulatory molecule ICOS regulates Th2 effector function in allergic airway disease. Recently, several studies with ICOS(-/-) mice have also demonstrated a role for ICOS in Th2 differentiation. To determine the effects of ICOS on the early immune response, we investigated augmenting ICOS costimulation in a Th2-mediated immune response to Schistosoma mansoni Ags. We found that augmenting ICOS costimulation with B7RP-1-Fc increased the accumulation of T and B cells in the draining lymph nodes postimmunization. Interestingly, the increased numbers were due in part to increased migration of undivided Ag-specific TCR transgenic T cells and surprisingly B cells, as well as non-TCR transgenic T cells. B7RP-1-Fc also increased the levels of the chemokines CCL21 and CXCL13 in the draining lymph node, suggesting ICOS costimulation contributes to migration by direct or indirect effects on dendritic cells, stromal cells and high endothelial venules. Further, the effects of B7RP-1-Fc were not dependent on immunization. Our data support a model in which ICOS costimulation augments the pool of lymphocytes in the draining lymph nodes, leading to an increase in the frequency of potentially reactive T and B cells.

ICOS controls the pool size of effector-memory and regulatory T cells

ICOS is an important regulator of T cell effector function. ICOS-deficient patients as well as knockout mice show severe defects in T cell-dependent B cell responses. Several in vitro and in vivo studies attributed this phenomenon to impaired up-regulation of cell surface communication molecules and cytokine synthesis by ICOS-deficient T cells. However, we now could show with Ag-specific T cells in a murine adoptive transfer system that signaling via ICOS does not significantly affect early T cell activation. Instead, ICOS substantially contributes to the survival and expansion of effector T cells upon local challenge with Ag and adjuvant. Importantly, the observed biological function of ICOS also extends to FoxP3+ regulatory T cells, as can be observed after systemic Ag delivery without adjuvant. In line with these findings, absence of ICOS under homeostatic conditions of nonimmunized mice leads to a reduced number of both effector-memory and FoxP3+ regulatory T cells. Based on these results, we propose a biological role for ICOS as a costimulatory, agonistic molecule for a variety of effector T cells with differing and partly opposing functional roles. This concept may reconcile a number of past in vivo studies with seemingly contradictory results on ICOS function.

Alternative activation is an innate response to injury that requires CD4+ T cells to be sustained during chronic infection

Alternatively activated macrophages (AAMPhi) are found in abundance during chronic Th2 inflammatory responses to metazoan parasites. Important roles for these macrophages are being defined, particularly in the context of Th2-mediated pathology and fibrosis. However, a full understanding of the requirements for alternative activation, particularly at the innate level, is lacking. We present evidence that alternative activation by the Th2 cytokines IL-4 and IL-13 is an innate and rapid response to tissue injury that takes place even in the absence of an infectious agent. This early response does not require CD4+ Th2 cells because it occurred in RAG-deficient mice. However, class II-restricted CD4+ T cell help is essential to maintain AAMPhi in response to infection, because AAMPhi were absent in RAG-deficient and MHC class II-deficient, but not B cell-deficient mice after chronic exposure to the nematode parasite, Brugia malayi. The absence of AAMPhi was associated with increased neutrophilia and reduced eosinophilia, suggesting that AAMPhi are involved in the clearance of neutrophils as well as the recruitment of eosinophils. Consistent with this hypothesis, AAMPhi show enhanced phagocytosis of apoptotic neutrophils, but not latex beads. Our data demonstrate that alternative activation by type 2 cytokines is an innate response to injury that can occur in the absence of an adaptive response. However, analogous to classical activation by microbial pathogens, Th2 cells are required for maintenance and full activation during the ongoing response to metazoan parasites.

The role of ICOS in the development of CD4 T cell help and the reactivation of memory T cells

We have addressed the role of the inducible costimulator (ICOS) in the development of T cell help for B cells and in the generation, survival and reactivation of memory CD4 T cells and B cells. We find that while T cell help for all antibody isotypes (including IgG2c) is impaired in ICOS knockout (ICOS-KO) mice, the IFN-γ response is little affected, indicating a defect in helper function that is unrelated to cytokine production. In addition, the ICOS-negative T cells do not accumulate in B cell follicles. Secondary (memory), but not primary, clonal proliferation of antigen-specific B cells is impaired in ICOS-KO mice, as is the generation of secondary antibody-secreting cells. Analysis of endogenous CD4 memory cells in ICOS-KO mice, using MHC class II tetramers, reveals normal primary clonal expansion, formation of memory clones and long-term (10 wk) survival of memory cells, but defective expansion upon reactivation in vivo. The data point to a role of ICOS in supporting secondary, memory and effector T cell responses, possibly by influencing cell survival. The data also highlight differences in ICOS dependency of endogenous T cell proliferation in vivo compared to that of adoptively transferred TCR-transgenic T cells.

B7RP-1-ICOS interactions are required for optimal infection-induced expansion of CD4+ Th1 and Th2 responses

Although initial reports linked the costimulatory molecule ICOS preferentially with the development of Th2 cells, there is evidence that it is not required for protective type 2 immunity to helminths and that it contributes to Th1 and Th2 responses to other parasites. To address the role of ICOS in the development of infection-induced polarized Th cells, ICOS(-/-) mice were infected with Trichuris muris or Toxoplasma gondii. Wild-type mice challenged with T. muris developed Th2 responses and expelled these helminths by day 18 postinfection, whereas ICOS(-/-) mice failed to clear worms and produced reduced levels of type 2 cytokines. However, by day 35 postinfection, ICOS(-/-) mice were able to mount an effective Th2 response and worms were expelled. This delay in protective immunity was associated with a defect in infection-induced increases in the number of activated and proliferating CD4+ T cells. Similarly, following challenge with T. gondii ICOS was required for optimal proliferation by CD4+ T cells. However, the reduced number of activated CD4+ T cells and associated defect in the production of IFN-gamma did not result in increased susceptibility to T. gondii, but rather resulted in decreased CNS pathology during the chronic phase of this infection. Taken together, these data are consistent with a model in which ICOS is not involved in dictating polarity of the Th response but rather regulates the expansion of these subsets.

Inducible costimulator: a modulator of IFN-gamma production in human tuberculosis

Effective host defense against Mycobacterium tuberculosis requires the induction of Th1 cytokine responses. We investigated the regulated expression and functional role of the inducible costimulator (ICOS), a receptor known to regulate Th cytokine production, in the context of human tuberculosis. Patients with active disease, classified as high responder (HR) or low responder (LR) patients according to their in vitro T cell responses against the Ag, were evaluated for T cell expression of ICOS after M. tuberculosis-stimulation. We found that ICOS expression significantly correlated with IFN-gamma production by tuberculosis patients. ICOS expression levels were regulated in HR patients by Th cytokines: Th1 cytokines increased ICOS levels, whereas Th2-polarizing conditions down-regulated ICOS in these individuals. Besides, in human polarized Th cells, engagement of ICOS increased M. tuberculosis IFN-gamma production with a magnitude proportional to ICOS levels on those cells. Moreover, ICOS ligation augmented Ag-specific secretion of the Th1 cytokine IFN-gamma from responsive individuals. In contrast, neither Th1 nor Th2 cytokines dramatically affected ICOS levels on Ag-stimulated T cells from LR patients, and ICOS activation did not enhance IFN-gamma production. However, simultaneous activation of ICOS and CD3 slightly augmented IFN-gamma secretion by LR patients. Together, our data suggest that the regulation of ICOS expression depends primarily on the response of T cells from tuberculosis patients to the specific Ag. IFN-gamma released by M. tuberculosis-specific T cells modulates ICOS levels, and accordingly, ICOS ligation induces IFN-gamma secretion. Thus, ICOS activation may promote the induction of protective Th1 cytokine responses to intracellular bacterial pathogens.

High resolution mapping of chromosomal regions controlling resistance to gastrointestinal nematode infections in an advanced intercross line of mice

Fine mapping of quantitative trait loci (QTL) associated with resistance to the gastrointestinal parasite Heligmosomoides polygyrus was achieved on F(6)/F(7) offspring (1076 mice) from resistant (SWR) and susceptible (CBA) mouse strains by selective genotyping (top and bottom 20% selected on total worm count in week 6). Fecal egg counts were recorded at weeks 2, 4, and 6, and the average was also analyzed. Blood packed cell volume in weeks 3 and 6 and five immunological traits (mucosal mast cell protease 1, granuloma score, IgG1 against adult worm, IgG1, and IgE to L4 antigen) were also recorded. On Chromosome 1 single-trait analyses identified a QTL with effects on eight traits located at about 24 cM on the F(2) mouse genome database (MGD) linkage map, with a 95% confidence interval (CI) of 20-32 cM established from a multitrait analysis. On Chromosome 17 a QTL with effects on nine traits was located at about 18 cM on the MGD map (CI 17.9-18.4 cM). Strong candidate genes for the QTL position on Chromosome 1 include genes known to be involved in regulating immune responses and on Chromosome 17 genes within the MHC, notably the Class II molecules and tumor necrosis factor.

Upregulation of ICOS on CD43+ CD4+ murine small intestinal intraepithelial lymphocytes during acute reovirus infection

Murine intestinal intraepithelial lymphocytes (IELs) can be classified according to expression of a CD43 glycoform recognized by the S7 monoclonal antibody. In this study, we examined the response of S7+ and S7- IELs in mice during acute reovirus serotype 3 (Dearing strain) infection, which was confirmed by virus-specific real-time PCR. In vivo proliferation increased significantly for both S7- and S7+ IELs on day 4 post-infection as determined by BrdU incorporation; however, expression of the inducible costimulatory (ICOS) molecule, which peaked on day 7 post-infection, was upregulated on S7+ CD4+ T cells, most of which were CD4+8- IELs. In vitro ICOS stimulation by syngeneic peritoneal macrophages induced IFN-gamma secretion from IELs from day 7 infected mice, and was suppressed by treatment with anti-ICOS mAb. Additionally, IFN-gamma mRNA increased in CD4+ IELs on day 6 post-infection. These findings indicate that S7- and S7+ IELs are differentially mobilized during the immune response to reovirus infection; that the regulated expression of ICOS is associated with S7+ IELs; and that stimulation of IELs through ICOS enhances IFN-gamma synthesis during infection.

Selective upregulation of immune regulatory and effector cytokine synthesis by intestinal intraepithelial lymphocytes following CD43 costimulation

The involvement of the CD43 molecule in the activation of mouse small intestinal intraepithelial lymphocytes (IELs) has been studied using a panel of twenty-two regulatory and effector immune response analytes. In the absence of stimulation in vitro, IELs produced low levels of CCL5 only. Upon CD3 stimulation, the activity of seven of twenty-two analytes was elevated relative to unstimulated cultures, including several proinflammatory cytokines and chemokines. Notably, CD3 stimulation in the presence of CD43 costimulation resulted in elevated levels of five analytes (interleukin-2, interferon-gamma, CCL5, granulocyte colony-stimulating factor, and granulocyte-monocyte colony-stimulating factor) above that produced by CD3 stimulation alone. That CD43 costimulation was responsible for elevated cytokine/chemokine activity was confirmed at the transcriptional level by real-time PCR for IFN-gamma and CCL5, and by ELISA for IFN-gamma. These findings open the way to a better understanding of the process by which T cells are activated in the intestinal epithelium.