IgE regulation and roles in asthma pathogenesis

Asthma and the predisposition to produce IgE are inherited as linked traits in families. In patients IgE levels correlate with asthma severity and bronchial hyperresponsiveness. The concept that IgE plays a critical role in asthma pathogenesis has driven the development of IgE blockers, which are currently being introduced into clinical use. This review focuses on the mechanisms whereby IgE participates both in immediate hypersensitivity responses in the airways and in the induction of chronic allergic bronchial inflammation. The molecular genetic events that give rise to IgE production by B cells and the cellular and cytokine factors that support IgE production in the bronchial mucosal microenvironment are discussed. It is clear that much remains to be learned regarding the roles of IgE in asthma and the genetic and environmental influences that lead to its production. Over the next few years, the emerging experience with anti-IgE in patients will provide a more complete understanding of the mechanisms whereby IgE contributes to disease, as well as the therapeutic potential of its inhibition.

Glucocorticoids upregulate CD40 ligand expression and induce CD40L-dependent immunoglobulin isotype switching

IL-4 and CD40 ligation are essential for IgE synthesis by B cells. We have shown previously that hydrocortisone (HC) induces IgE synthesis in IL-4-stimulated human B cells. In this study we demonstrate that HC fails to induce IgE synthesis in B cells from CD40 ligand-deficient (CD40L-deficient) patients. Disruption of CD40L-CD40 interactions by soluble CD40-Ig fusion protein or anti-CD40L mAb blocked the capacity of HC to induce IgE synthesis in normal B cells. HC upregulated CD40L mRNA expression in PBMCs and surface expression of CD40L in PBMCs as well as in purified populations of T and B cells. Upregulation of CD40L mRNA in PBMCs occurred 3 hours after stimulation with HC and was inhibited by actinomycin D. Upregulation of CD40L mRNA and induction of IgE synthesis by HC were inhibited by the steroid hormone receptor antagonist RU-486. These results indicate that ligand-mediated activation of the glucocorticoid receptor upregulates CD40L expression in human lymphocytes.

An obligate role for T-cell receptor alphabeta+ T cells but not T-cell receptor gammadelta+ T cells, B cells, or CD40/CD40L interactions in a mouse model of atopic dermatitis

BACKGROUND

We recently described a murine model of atopic dermatitis (AD) elicited by epicutaneous sensitization with ovalbumin (OVA). The skin lesions in these mice were characterized by a dermal infiltrate consisting of eosinophils and T cells and by increased expression of the TH2 cytokines IL-4 and IL-5. Epicutaneous sensitization induces a rise in the levels of serum total IgE and OVA-specific antibodies, further indicating that it elicits a predominantly TH2 response.

OBJECTIVE

This study was undertaken to assess the roles of T cells, B cells, and CD40L-CD40 interactions in AD.

METHODS

Mice with targeted gene deletions were sensitized with OVA. Histologic and immunohistochemical examinations, as well as measurements of IL-4 mRNA, were performed on OVA-sensitized skin. Total and antigen-specific serum IgE levels were determined.

RESULTS

RAG2(-/-) mice, which lack both T and B cells, did not exhibit cellular infiltration, induction of dermal IL-4 mRNA, or elevation of serum IgE after OVA sensitization; all of these features were present in B-cell-deficient IgH(-/-) mice. T-cell receptor alpha(-/-) mice did not display cellular infiltration, IL-4 mRNA expression, or increased IgE levels after OVA sensitization, but these responses were elicited in T-cell receptor delta(-/-) mice after sensitization. Absence of CD40 had no effect on these responses.

CONCLUSION

These results suggest that alphabeta T cells, but not gammadelta T cells, B cells, or CD40L-CD40 interactions, are critical for skin inflammation and the TH2 response in AD.

Roles of SLP-76, phosphoinositide 3-kinase, and gelsolin in the platelet shape changes initiated by the collagen receptor GPVI/FcR gamma-chain complex

How platelet shape change initiated by a collagen-related peptide (CRP) specific for the GPVI/FcR gamma-chain complex (GPVI/FcR gamma-chain) is coupled to SLP-76, phosphoinositide (PI) 3-kinase, and gelsolin is reported. As shown by video microscopy, platelets rapidly round and grow dynamic filopodial projections that rotate around the periphery of the cell after they contact a CRP-coated surface. Lamellae subsequently spread between the projections. All the actin-driven shape changes require SLP-76 expression. SLP-76 is essential for the Ca(++) mobilization induced by CRP, whereas PI 3-kinase only modulates it. The extension of lamellae requires net actin assembly and an exposure of actin filament barbed ends downstream of PI 3-kinase. Gelsolin expression is also required for the extension of lamellae, but not for the formation of filopodia. Altogether, the data describe the role of SLP-76 in the platelet activation initiated by GPVI/FcR gamma-chain and the roles of PI 3-kinase and gelsolin in lamellae spreading. (Blood. 2000;96:3786-3792)

CD40L, but not CD40, is required for allergen-induced bronchial hyperresponsiveness in mice

Asthma is characterized by immunoglobulin (Ig) E production, infiltration of the respiratory mucosa by eosinophils (EOSs) and mononuclear cells, and bronchial hyperresponsiveness (BHR). Interaction of CD40 on B cells and antigen presenting cells, with its ligand (CD40L) expressed transiently on activated T cells, is known to augment both T cell-driven inflammation and humoral immune responses, especially IgE production. Considering both the prominent role of inflammation in asthma and the association of the disease with IgE, we hypothesized that CD40-CD40L interactions would be important in pathogenesis. To test this hypothesis, we subjected wild-type (WT) mice and animals lacking either CD40 or CD40L to repeated inhalation of Aspergillus fumigatus (Af ) antigen. Af-treated WT mice displayed elevated IgE levels, bronchoalveolar lavage and pulmonary tissue eosinophilic inflammation, and BHR. IgE production was markedly suppressed in both the CD40 -/- and CD40L -/- strains. However, pulmonary inflammation did not appear to be inhibited by either of these mutations. Paradoxically, development of BHR was prevented by the lack of CD40L but not by the absence of CD40. We conclude that CD40/CD40L interactions, although critical in the induction of IgE responses to inhaled allergen, are not required for the induction of EOS-predominant inflammation. CD40L, but not CD40, is necessary for the development of allergen-induced BHR.

Identification and characterization of two CD40-inducible enhancers in the mouse TRAF1 gene locus

We have shown that CD40 engagement induces TRAF1 gene expression in B lymphocytes. Here we report that CD40-dependent TRAF1 gene transcription in murine B cells is controlled by two enhancer regions. One region is located approximately 2 kb upstream of the transcription start site and the other lies in the intron between exons 5 and 6. The upstream enhancer contains a single NF-kappaB site in addition to sites that bind constitutive transcription factors. Mutation of this NF-kappaB site completely abrogates CD40-driven TRAFl transcription. The intronic enhancer contains two sites that strongly bind the CD40-inducible factors NF-kappaB and AP-1. Simultaneous mutation of the AP-1 site and of the NF-kappaB site abolishes transcription driven by this enhancer. When cloned together into reporter constructs, the two TRAF1 enhancers do not synergize, suggesting that each enhancer may separately participate in the induction of TRAF1 transcription in B cells following CD40 activation.

Multicenter, double-blind, placebo-controlled, multiple-challenge evaluation of reported reactions to monosodium glutamate

BACKGROUND

The frequency of reactions reported to occur after the consumption of monosodium glutamate (MSG) is the subject of controversy.

OBJECTIVE

We conducted a multicenter, multiphase, double-blind, placebo-controlled study with a crossover design to evaluate reactions reportedly caused by MSG.

METHODS

In 3 of 4 protocols (A, B, and C), MSG was administered without food. A positive response was scored if the subject reported 2 or more symptoms from a list of 10 symptoms reported to occur after ingestion of MSG-containing foods within 2 hours. In protocol A 130 self-selected reportedly MSG-reactive volunteers were challenged with 5 g of MSG and with placebo on separate days (days 1 and 2). Of the 86 subjects who reacted to MSG, placebo, or both in protocol A, 69 completed protocol B to determine whether the response was consistent and dose dependent. To further examine the consistency and reproducibility of reactions to MSG, 12 of the 19 subjects who responded to 5 g of MSG but not to placebo in both protocols A and B were given, in protocol C, 2 challenges, each consisting of 5 g of MSG versus placebo.

RESULTS

Of 130 subjects in protocol A, 50 (38. 5%) responded to MSG only, 17 (13.1%) responded to placebo only (P <. 05), and 19 (14.6%) responded to both. Challenge with increasing doses of MSG in protocol B was associated with increased response rates. Only half (n = 19) of 37 subjects who reacted to 5 g of MSG but not placebo in protocol A reacted similarly in protocol B, suggesting inconsistency in the response. Two of the 19 subjects responded in both challenges to MSG but not placebo in protocol C; however, their symptoms were not reproducible in protocols A through C. These 2 subjects were challenged in protocol D 3 times with placebo and 3 times with 5 g of MSG in the presence of food. Both responded to only one of the MSG challenges in protocol D.

CONCLUSION

The results suggest that large doses of MSG given without food may elicit more symptoms than a placebo in individuals who believe that they react adversely to MSG. However, neither persistent nor serious effects from MSG ingestion are observed, and the responses were not consistent on retesting.

The role of SLP-76 and LAT in lymphocyte development

SLP-76 and LAT are two recently identified adapter proteins that are involved in the signal transduction cascade initiated by engagement of the TCR. The role of these two molecules in thymocyte development has become clearer following studies of gene targeted mice. The data indicate that SLP-76 and LAT are each critical for the expansion and differentiation of double-negative thymocytes and that SLP-76 is essential for allelic exclusion at the TCRbeta locus.

Review of alleged reaction to monosodium glutamate and outcome of a multicenter double-blind placebo-controlled study

Monosodium glutamate (MSG) has a long history of use in foods as a flavor enhancer. In the United States, the Food and Drug Administration has classified MSG as generally recognized as safe (GRAS). Nevertheless, there is an ongoing debate exists concerning whether MSG causes any of the alleged reactions. A complex of symptoms after ingestion of a Chinese meal was first described in 1968. MSG was suggested to trigger these symptoms, which were referred to collectively as Chinese Restaurant Syndrome. Numerous reports, most of them anecdotal, were published after the original observation. Since then, clinical studies have been performed by many groups, with varying degrees of rigor in experimental design ranging from uncontrolled open challenges to double-blind, placebo controlled (DBPC) studies. Challenges in subjects who reported adverse reactions to MSG have included relatively few subjects and have failed to show significant reactions to MSG. Results of surveys and of clinical challenges with MSG in the general population reveal no evidence of untoward effects. We recently conducted a multicenter DBPC challenge study in 130 subjects (the largest to date) to analyze the response of subjects who report symptoms from ingesting MSG. The results suggest that large doses of MSG given without food may elicit more symptoms than a placebo in individuals who believe that they react adversely to MSG. However, the frequency of the responses was low and the responses reported were inconsistent and were not reproducible. The responses were not observed when MSG was given with food.

Adapter proteins SLP-76 and BLNK both are expressed by murine macrophages and are linked to signaling via Fcgamma receptors I and II/III

The SLP-76 (Src homology 2 domain-containing leukocyte protein of 76 kDa) adapter protein is expressed in T cells and myeloid cells, whereas its homologue BLNK (B cell linker protein) is expressed in B cells. SLP-76 and BLNK link immunoreceptor tyrosine-based activation motif-containing receptors to signaling molecules that include phospholipase C-gamma, mitogen-activated protein kinases, and the GTPases Ras and Rho. SLP-76 plays a critical role in T cell receptor, FcvarepsilonRI and gpVI collagen receptor signaling, and participates in signaling via FcgammaR and killer cell inhibitory receptors. BLNK plays a critical role in B cell receptor signaling. We show that murine bone marrow-derived macrophages express both SLP-76 and BLNK. Selective ligation of FcgammaRI and FcgammaRII/III resulted in tyrosine phosphorylation of both SLP-76 and BLNK. SLP-76(-/-) bone marrow-derived macrophages display FcgammaR-mediated tyrosine phosphorylation of Syk, phospholipase C-gamma2, and extracellular signal regulated kinases 1 and 2, and normal FcgammaR-dependent phagocytosis. These data suggest that both SLP-76 and BLNK are coupled to FcgammaR signaling in murine macrophages.

Molecular mechanisms of IgE regulation

IgE antibody plays an important role in allergic diseases. IgE synthesis by B cells requires two signals. The first signal is delivered by the cytokines IL-4 or IL-13, which target the Cepsilon gene for switch recombination. The second signal is delivered by interaction of the B cell surface antigen CD40 with its ligand (CD40L) expressed on activated T cells. This activates deletional switch recombination. We review the molecular mechanisms of IL-4 and CD40 signaling that lead to IgE isotype switching and discuss the implications for intervening to abort or suppress the IgE antibody response.

Allelic exclusion of the T cell receptor beta locus requires the SH2 domain-containing leukocyte protein (SLP)-76 adaptor protein

Signaling via the pre-T cell receptor (TCR) is required for the proliferative expansion and maturation of CD4(-)CD8(-) double-negative (DN) thymocytes into CD4(+)CD8(+) double-positive (DP) cells and for TCR-beta allelic exclusion. The adaptor protein SH2 domain-containing leukocyte protein (SLP)-76 has been shown to play a crucial role in thymic development, because thymocytes of SLP-76(-/-) mice are arrested at the CD25(+)CD44(-) DN stage. Here we show that SLP-76(-/-) DN thymocytes express the pre-TCR on their surfaces and that introduction of a TCR-alpha/beta transgene into the SLP-76(-/-) background fails to cause expansion of DN thymocytes or developmental progression to the DP stage. Moreover, analysis of TCR-beta rearrangement in SLP-76(-/-) TCR-transgenic mice or in single CD25(+)CD44(-) DN cells from SLP-76(-/-) mice indicates an essential role of SLP-76 in TCR-beta allelic exclusion.

Ku in the cytoplasm associates with CD40 in human B cells and translocates into the nucleus following incubation with IL-4 and anti-CD40 mAb

CD40 plays a critical role in survival, growth, differentiation, and class switching of B lymphocytes. Although Ku is required for immunoglobulin class switching, how CD40 signal transduction is coupled to Ku is still unknown. Here, we show that CD40 directly interacts with Ku through the membrane-proximal region of cytoplasmic CD40. Ku was confined to the cytoplasm in human primary B cells, and the engagement of CD40 on the B cells cultured in the presence of IL-4 resulted in the dissociation of Ku from CD40, translocation of Ku into the nucleus, and increase in the activity of DNA-dependent protein kinase. These findings indicate that Ku is involved in the CD40 signal transduction pathway and may play an important role in the CD40-mediated events.

SLP-76 deficiency impairs signaling via the high-affinity IgE receptor in mast cells

SLP-76 is an adapter protein expressed in T cells and myeloid cells that is a substrate for ZAP-70 and Syk. SLP-76-deficient mice exhibit a profound block in T-cell development. We found that although SLP-76 is expressed in mouse mast cells, SLP-76(-/-) mice have normal numbers of mast cells in their skin and bronchi. SLP-76(-/-) mice are resistant to IgE-mediated passive anaphylaxis. SLP-76(-/-) mice sensitized with IgE anti-dinitrophenyl (DNP) and then challenged with DNP-HSA developed only mild and transient tachycardia, failed to increase their plasma histamine level, and all survived the antigen challenge. Bone marrow-derived mast cells (BMMCs) from SLP76(-/-) mice failed to release beta-hexosaminidase and to secrete IL-6 after FcepsilonRI cross-linking. Tyrosine phosphorylation of phospholipase C-gamma1 (but not of Syk) and calcium mobilization in response to IgE cross-linking were reduced in SLP-76-deficient BMMCs. These results suggest that SLP-76 plays an important role in FcepsilonRI-mediated signaling in mast cells.

The human WASP-interacting protein, WIP, activates the cell polarity pathway in yeast

WIP, the Wiskott-Aldrich syndrome protein-interacting protein, is a human protein involved in actin polymerization and redistribution in lymphoid cells. The mechanism by which WIP reorganizes actin cytoskeleton is unknown. WIP is similar to yeast verprolin, an actin- and myosin-interacting protein required for polarized morphogenesis. To determine whether WIP and verprolin are functional homologues, we analyzed the function of WIP in yeast. WIP suppresses the growth defects of VRP1 missense and null mutations as well as the defects in cytoskeletal organization and endocytosis observed in vrp1-1 cells. The ability of WIP to replace verprolin is dependent on its WH2 actin binding domain and a putative profilin binding domain. Immunofluorescence localization of WIP in yeast cells reveals a pattern consistent with its function at the cortical sites of growth. Thus, like verprolin, WIP functions in yeast to link the polarity development pathway and the actin cytoskeleton to generate cytoskeletal asymmetry. A role for WIP in cell polarity provides a framework for unifying, under a common paradigm, distinct molecular defects associated with immunodeficiencies like Wiskott-Aldrich syndrome.

Structure of the murine TRAF1 gene

We have cloned, characterized and sequenced the murine TNF Receptor Associated Factor 1 (TRAF1) gene. Restriction mapping and Southern blotting analysis revealed that the TRAF1 gene comprises 10 exons and 9 intervening introns and spreads over 18 kb of genomic DNA. 5'-RACE analysis of the TRAF1 transcript using mRNA from activated spleen B cells revealed several transcription start sites between positions -42 to +4 relative to the 5'end of the murine TRAF1 cDNA sequence. We also isolated and sequenced the 5'-upstream promoter region, which lacks TATA-like and CAAT-like sites but contains GC-rich sequences. Taken together, these results suggest that the TRAF1 gene promoter is a member of the class of Sp-1-dependent promoters. Near the transcription initiation start site we identified three identical decanucleotide repeats (CCAGCCCAGC) which may play a role in the transcriptional regulation of TRAF1 expression. In addition we show that TRAF1 mRNA is not expressed in non-stimulated lymphocytes but can be induced upon activation with different stimuli, including anti-CD3, anti-IgM, anti-CD40 antibodies, LPS, or a combination of phorbol-12-myristate-13-acetate and ionomycin.

Contribution of CD40-CD154-mediated costimulation to an alloresponse in vivo

BACKGROUND

Costimulation through CD40-CD154 plays an important role in T-cell activation. Although systemic administration of anti-CD154 antibody prevents or delays rejection of organ allografts in animal models, the molecular mechanisms responsible for this effect are not well defined.

METHODS

We have previously demonstrated that priming of mice (H2d) with CD40-/- but not with wildtype naive B cells (H2b) leads to alloantigen-specific T-cell hyporesponsiveness in vitro. In the present study, we investigated whether such priming modifies allograft rejection in a major histocompatibility complex-mismatched murine cardiac transplantation model.

RESULTS

Priming of hosts with donor-specific CD40-/- B cells delayed rejection of subsequently transplanted wild-type cardiac allografts by 8.0 days (P<0.001). The lack of CD40 on the cardiac graft delayed rejection in unprimed or primed hosts by 3-5 days. Prolongation of graft survival correlated with the failure of infused CD40-/- B cells to express B7.2 and ICAM-1 in vivo.

CONCLUSIONS

Our data suggest that CD40-CD154 costimulation contributes to T cell priming to alloantigens in vivo and to a second set rejection phase in which donor antigens are presented to primed T cells.

Effect of intravenous immunoglobulin on steroid consumption in patients with severe asthma: a double-blind, placebo-controlled, randomized trial

BACKGROUND

There is a significant group of patients with severe asthma who require chronic use of systemic steroids for control of their disease. These patients are at risk for severe side effects from oral steroids. Intravenous immunoglobulin (IVIG) has immunomodulatory properties, and a few open-label trials have suggested its possible benefit in individuals with severe asthma.

OBJECTIVE

This study was designed to assess the potential benefit of IVIG as a steroid-sparing agent in patients with severe asthma.

METHODS

Thirty-eight immunocompetent steroid-requiring patients with severe asthma were randomly enrolled in a double-blind, placebo-controlled trial of IVIG.

RESULTS

Of the 38 patients enrolled, 28 patients completed the study. A significant reduction in oral steroid requirement was observed in both the IVIG-treated (n = 16) and the placebo-treated (n = 12) patients. Further exploration of the results showed that IVIG, but not placebo, had a significant steroid-sparing effect in patients requiring high doses of oral steroids (ie, >2000 mg in the year before the study). Within this subgroup, IVIG treatment (n = 9) resulted in a significant decrease in oral steroid requirement, with a median of 16.4 mg/day during the pretreatment period to 3 mg/day during the treatment phase (P =. 0078). No significant decrease in oral steroid requirement was observed in placebo-treated patients (n = 8) within this subgroup. Objective and subjective parameters of the patients' asthma were unchanged in spite of the steroid tapering achieved in the group treated with IVIG.

CONCLUSION

IVIG may be a useful steroid-sparing agent in patients with severe asthma requiring high doses of oral steroids.

Roles of TH1 and TH2 cytokines in a murine model of allergic dermatitis

Skin lesions in atopic dermatitis (AD) are characterized by hypertrophy of the dermis and epidermis, infiltration by T cells and eosinophils, and expression of the cytokines IL-4, IL-5, and IFN-gamma. The role of these cytokines in the pathogenesis of AD is not known. We took advantage of a recently described murine model of AD elicited by epicutaneous sensitization with ovalbumin (OVA) (1) and of the availability of mice with targeted deletions of the IL-4, IL-5, and IFN-gamma cytokine genes to assess the role of these cytokines in this model.OVA-sensitized skin from IL-5(-/-) mice had no detectable eosinophils and exhibited decreased epidermal and dermal thickening. Sensitized skin from IL-4(-/-) mice displayed normal thickening of the skin layers but had a drastic reduction in eosinophils and a significant increase in infiltrating T cells. These findings were associated with a reduction in eotaxin mRNA and an increase in mRNA for the T-cell chemokines macrophage inflammatory protein-2 (MIP-2), MIP-1beta, and RANTES. Sensitized skin from IFN-gamma-/- mice was characterized by reduced dermal thickening. These results suggest that both the TH2 cytokines IL-4 and IL-5 and the TH1 cytokine IFN-gamma play important roles in the inflammation and hypertrophy of the skin in AD.

Waltzing with WASP

Wiskott-Aldrich syndrome (WAS) is an inherited immune deficiency that is marked by eczema, bleeding and recurrent infections. The lymphocytes and platelets of WAS patients display cytoskeletal abnormalities, and their T lymphocytes show a diminished proliferative response to stimulation through the T-cell receptor-CD3 complex (TCR-CD3). The product of the WAS gene, WAS protein (WASP), binds to the small GTPase Cdc42. Small GTPases of the Rho family are crucial for the regulation of the actin-based cytoskeleton. WASP and its relative NWASP might play an important role in regulating the actin cytoskeleton. Since both WASP and NWASP have the potential to bind to multiple proteins, they might serve as a hub to coordinate the redistribution of many cellular signals to the actin cytoskeleton. In this review, the authors discuss the possible role of WASP/NWASP and of the newly described protein WIP, which interacts with WASP and NWASP, in coupling signals from the T-cell receptor to the actin-based cytoskeleton.

Regulation of IgE synthesis: the molecular basis and implications for clinical modulation

The reagenic antibody in the sera of atopic individuals, first described by Prausnitz and Kustner and later determined to be Immunoglobulin (Ig) E by the Ishizakas, plays a critical role in the pathogenesis of allergic disease. Investigation into the cellular basis of IgE regulation has provided important insights into a disease process that affects up to 30% of the population world-wide. Over the last decade, the molecular events regulating IgE synthesis have been actively investigated. In this review, we will discuss the various components of this system including the cells, cytokines, signal transduction events, and molecular mechanisms that participate in human IgE synthesis and explore rational therapeutic approaches directed at the modulation of these systems.

The role of the CD40 pathway in alloantigen-induced hyporesponsiveness in vivo

Resting B (rB) cells are known to be incompetent APCs in vitro, which alone can induce specific unresponsiveness to single minor histocompatibility (miH) Ags and, when combined with CD40 pathway blockade, can induce hyporesponsiveness to MHC molecules in vivo. Here we show that anti-CD40 ligand (CD40L) mAb does not prevent the expression of B7-2 on allogeneic rB cells in vivo but did prolong donor-specific cardiac allograft survival. Moreover, pretreatment with professional APCs combined with anti-CD40L mAb induced hyporesponsiveness to alloantigens in vivo. rB cells from CD40 knockout mice were unable to induce unresponsiveness, while graft prolongation was achieved in CD40L knockout recipients pretreated with wild-type rB cells. These data suggest that CD40-CD40L interactions in the recipient play a critical role in the induction of hyporesponsiveness to alloantigens in vivo and that the effect of the CD40 pathway may be independent of its effect on the B7 costimulatory pathway.

The Role of the CD40 Pathway in Alloantigen-Induced Hyporesponsiveness In Vivo

Resting B (rB) cells are known to be incompetent APCs in vitro, which alone can induce specific unresponsiveness to single minor histocompatibility (miH) Ags and, when combined with CD40 pathway blockade, can induce hyporesponsiveness to MHC molecules in vivo. Here we show that anti-CD40 ligand (CD40L) mAb does not prevent the expression of B7-2 on allogeneic rB cells in vivo but did prolong donor-specific cardiac allograft survival. Moreover, pretreatment with professional APCs combined with anti-CD40L mAb induced hyporesponsiveness to alloantigens in vivo. rB cells from CD40 knockout mice were unable to induce unresponsiveness, while graft prolongation was achieved in CD40L knockout recipients pretreated with wild-type rB cells. These data suggest that CD40-CD40L interactions in the recipient play a critical role in the induction of hyporesponsiveness to alloantigens in vivo and that the effect of the CD40 pathway may be independent of its effect on the B7 costimulatory pathway.