Weak TCR stimulation induces a calcium signal that triggers IL-4 synthesis, stronger TCR stimulation induces MAP kinases that control IFN-gamma production

Th1 and Th2 cells produce different cytokines and have distinct functions. Th1/Th2 cell differentiation is influenced, among other factors, by the nature of TCR-MHC interactions. However, how the TCR transduces a signal resulting in IFN-gamma or IL-4 production is a matter of debate. For example, some authors reported a loss of calcium signaling pathway in Th2 cells. We used a T cell hybridoma producing IL-4 upon weak TCR stimulation and both IL-4 and IFN-gamma for strong TCR engagement as a model to study how TCR signaling pathways are differentially activated in both conditions of stimulation and how this influences the production of cytokines. We show that: (1) the calcium response is identical following weak and strong TCR stimulation; (2) mitogen-activated protein kinase(MAPK) activation is a gradual phenomenon depending upon the strength of TCR activation; (3) a calcium response, even weak, triggers IL-4 expression; (4) IFN-gamma synthesis requires not only a calcium response but also MAPK activation. The MAPK pathway is dispensable for IL-4 production, although it amplifies IL-4 synthesis upon strong TCR stimulation; (5) TCR-induced IL-4 production also depends on calcium signaling in Th2 cells, while IFN-gamma synthesis is dependent, in addition, on MAPK activation in Th1 cells.

Selective activation and expansion of high-affinity CD4+ T cells in resistant mice upon infection with Leishmania major

Using multimers of MHC class II molecules linked to a peptide derived from the Leishmania LACK antigen, we have compared the fate of parasite-specific CD4+ T cells in resistant and susceptible mice transgenic for the beta chain of a LACK-specific TCR. Activated T cells were readily detected in the draining lymph nodes of infected animals. Although the kinetics of activation and expansion were similar in both strains, T cells from susceptible and resistant mice expressed low- and high-affinity TCR, respectively. As T cells from resistant mice produced more IFN-gamma and less IL-4 than those from susceptible animals, our results suggest that differences in TCR usage between MHC-matched animals may influence the development of the antiparasite immune response.

Dendritic cells prime in vivo alloreactive CD4 T lymphocytes toward type 2 cytokine- and TGF-beta-producing cells in the absence of CD8 T cell activation

The mechanisms that influence the polarization of CD4 T cells specific for allogeneic MHC class II molecules in vivo are still poorly understood. We have examined the pathway of alloreactive CD4 T cell differentiation in a situation in which only CD4 T cells could be activated in vivo. In this report we show that priming of adult mice with allogeneic APC, in the absence of MHC class I-T cell interactions, induces a strong expansion of type 2 cytokine-producing allohelper T cells. These alloantigen-specific CD4 T cells directly recognize native allogeneic MHC class II molecules on APC and secrete, in addition to the prototypic Th2 cytokines IL-4, IL-5, and IL-10, large amounts of TGF-beta. The default Th2-phenotype acquisition is not genetically controlled and occurred both in BALB/c and C57BL/6 mice. CD8 T cells are the principal cell type that controls CD4 T cell differentiation in vivo. Furthermore, we demonstrate that strong Th2 priming can be induced not only with allogeneic splenocytes but also with a low number of bone marrow-derived dendritic cells. Finally, using a passive transfer system, we provide direct evidence that CD8 T cell expansion in situ promotes alloreactive Th1 cell development principally by preventing their default development to the Th2 pathway in a mechanism that is largely IFN-gamma independent. Therefore, this work demonstrates that type 2 cytokine production represents a dominant pathway of alloreactive CD4 T cell differentiation in adult mice, a phenomenon that was initially thought to occur only during the neonatal period.

Lethal host-versus-graft disease and hypereosinophilia in the absence of MHC I-T-cell interactions

Neonatal injection of semiallogeneic spleen cells in BALB/c mice induces a self-limited state of chimerism that promotes the differentiation of donor-specific CD4 T cells toward the Th2 phenotype. Here we show that injection of spleen cells from beta2-microglobulin-deficient (BALB/c x C57BL/6) F1 mice into BALB/c newborns with a disrupted beta2-microglobulin (beta2m) gene results in a lethal lymphoproliferative disorder associated with uncontrolled Th2 response, long-term persistence of donor B cells, and sustained blood eosinophilia. Autoimmune manifestations are also enhanced and characterized by a severe autoantibody-mediated glomerulonephritis. Histological examination of the spleen shows a hyperplasia of periarteriolar lymphoid sheaths, with accumulation of eosinophils and basophils, and variable degree of fibrosis. Perivascular lymphoid infiltrates with eosinophils are also found in the lung and are correlated with disease severity. Such abnormalities are almost absent using beta2m-sufficient mice. These data demonstrate that induction of lymphoid chimerism in the absence of MHC class I-T-cell interactions results in a lethal form of host-versus-graft disease that represents a unique model of Th2-dependent chronic inflammatory disease associated with an hypereosinophilic syndrome in mice.

Experimental autoimmune myasthenia gravis may occur in the context of a polarized Th1- or Th2-type immune response in rats

Experimental autoimmune myasthenia gravis (EAMG) is a T cell-dependent, Ab-mediated autoimmune disease induced in rats by a single immunization with acetylcholine receptor (AChR). Although polarized Th1 responses have been shown to be crucial for the development of mouse EAMG, the role of Th cell subsets in rat EAMG is not well established. In the present work we show that while the incidence and severity of EAMG are similar in Lewis (LEW) and Brown-Norway (BN) rats, strong differences are revealed in the immune response generated. Ag-specific lymph node cells from LEW rats produced higher amounts of IL-2 and IFN-gamma than BN lymph node cells, but expressed less IL-4 mRNA. IgG1 and IgG2b anti-AChR isotype predominated in BN and LEW rats, respectively, confirming the dichotomy of the immune response observed between the two strains. Furthermore, although IL-12 administration or IFN-gamma neutralization strongly influenced the Th1/Th2 balance in BN rats, it did not affect the disease outcome. These data demonstrate that a Th1-dominated immune response is not necessarily associated with disease severity in EAMG, not only in rats with disparate MHC haplotype but also in the same rat strain, and suggest that in a situation where complement-fixing Ab can be generated as a consequence of either Th1- or Th2-mediated T cell help, deviation of the immune response will not be an adequate strategy to prevent this Ab-mediated autoimmune disease.

Flow cytometric analysis of intracellular interferon-gamma synthesis in rat CD4 T cells

To date the techniques used to analyse cytokine expression by rat T cells do not give information about the simultaneous production of different cytokines from individual cells. Recently, a method for analysing the intracellular production of cytokines at the single cell level using flow cytometry has been developed. It is well established that the most critical requirement for successful intracellular cytokine staining is the availability of appropriate antibodies. In rat, it is possible to stain for intracellular IL-4 and IL-10 (Th2 cytokines) using the commercially available antibodies but not for Th1 cytokines. In the present work, we show that DB1, a mouse anti-rat IFN-gamma monoclonal antibody, could be used for intracytoplasmic staining of IFN-gamma producing rat CD4 T cells. The specificity of the staining was confirmed using a molar excess of unlabelled antibodies or recombinant cytokine. Finally, intracellular staining for IFN-gamma correlates with cytokine production in culture supernatant as evaluated by ELISA analysis.

Beta 2-microglobulin-dependent T cells are not necessary for alloantigen-induced Th2 responses after neonatal induction of lymphoid chimerism in mice

We have analyzed the requirement for beta 2-microglobulin (beta 2m)-dependent T cells in the generation of allogeneic Th2 responses in vivo. A neonatal injection of semiallogeneic cells in BALB/c mice induces a state of chimerism that promotes the differentiation of donor-specific CD4+ T cells toward the Th2 phenotype. Polyclonal T-B cell interactions occur in this model between host Th2 and donor B cells, resulting in the production of IgE Abs. IgE production and Th2-priming are critically dependent upon the early production of IL-4. Our data in the present paper demonstrate that: 1) IgE synthesis and the up-regulation of MHC class II and CD23 molecules on B cells are independent of beta 2m expression in the host, 2) no difference in the induction of CD4 alloreactive Th2 cells could be observed between beta 2m-/- and their wild-type control littermates when Th2-priming was measured in adult mice, and 3) the Th2 response and IgE production is induced in the complete absence of beta 2m-dependent T cells both in the host and in the inoculum. Therefore, using a variety of assays, we could not demonstrate diminished responses in mice with a disrupted beta 2m gene in this model of Th2-mediated allogeneic interaction, indicating that beta 2m-dependent NK1.1+ and CD8+ T cells are not required for the generation of alloreactive Th2 responses in vivo.

Regulation of the IL-12 receptor beta2 subunit by soluble antigen and IL-12 in vivo

Continuous administration of soluble protein antigen to BALB/c mice inhibits the development of Th1 and induces selective differentiation of Th2 cells. Here we show that interleukin (IL)-12, administered together with soluble protein through a mini-osmotic pump implanted subcutaneously, not only prevents the inhibition of Th1 cell development, but stimulates higher interferon (IFN)-gamma production than in mice receiving IL-12 alone. In parallel to co-stimulation of Th1 cell development, co-administration of IL-12 blocks the Th2 response induced by soluble protein. IL-12 administered in adjuvant with antigen or intraperitoneally 2 days after the immunization does not break the inhibition of Th1 but can still decrease the Th2 response induced by pretreatment with soluble protein antigen. In contrast to IL-12, co-administration of IL-2 or IFN-gamma does not affect the diversion to Th2 induced by soluble antigen. Thus IL-12, but not IL-2 nor IFN-gamma, converts in vivo the inhibitory signal for Th1 cell development delivered by soluble antigen into an immunogenic one, while blocking a positive signal for Th2 cell differentiation. A molecular basis for the co-stimulation of Th1 priming and the prevention of Th2 differentiation by IL-12 in vivo is provided by the observation that transcripts encoding the IL-12 receptor beta2 chain, which is required for IL-12 signaling and Th1 cell development, are selectively inhibited by soluble antigen but are enhanced by IL-12 co-administration.

Non-MHC-linked Th2 cell development induced by soluble protein administration predicts susceptibility to Leishmania major infection

Continuous administration of soluble protein Ag followed by immunization with the same Ag in adjuvant results in the selective development of Ag-specific CD4+ Th2 cells in both normal and beta2-microglobulin-deficient BALB/c mice. In addition to chronic administration by mini-osmotic pump, single bolus i.p., but not i.v., injection of protein Ag induces Th2 cell expansion. Strong Th2 cell priming depends on a non-MHC-linked genetic polymorphism. It is observed in all congenic strains on BALB background tested, BALB/c, BALB/b, and BALB/k, but not in MHC-matched strains on disparate genetic background, B10.D2, C57BL/6, and C3H. DBA/2 mice appear to have an intermediate phenotype, as shown by their weaker capacity to mount Th2 responses as compared with BALB/c mice after soluble Ag administered by either mini-osmotic pumps or single bolus i.p. Conversely, induction of Th1 cell unresponsiveness by soluble protein is observed in any mouse strain tested, following any mode of Ag administration. These data demonstrate that non-MHC-linked genetic polymorphism controls the priming of Th2 but not the inhibition of Th1 cells induced by administration of soluble protein. The pattern of Th2 responses in these different strains is predictive of disease outcome following Leishmania major infection and supports the hypothesis that systemic Ag presentation in the absence of strong inflammatory signals may represent an important stimulus leading to selective Th2 cell development in susceptible mouse strains.

Non-MHC-linked Th2 cell development induced by soluble protein administration predicts susceptibility to Leishmania major infection

Continuous administration of soluble protein Ag followed by immunization with the same Ag in adjuvant results in the selective development of Ag-specific CD4+ Th2 cells in both normal and beta2-microglobulin-deficient BALB/c mice. In addition to chronic administration by mini-osmotic pump, single bolus i.p., but not i.v., injection of protein Ag induces Th2 cell expansion. Strong Th2 cell priming depends on a non-MHC-linked genetic polymorphism. It is observed in all congenic strains on BALB background tested, BALB/c, BALB/b, and BALB/k, but not in MHC-matched strains on disparate genetic background, B10.D2, C57BL/6, and C3H. DBA/2 mice appear to have an intermediate phenotype, as shown by their weaker capacity to mount Th2 responses as compared with BALB/c mice after soluble Ag administered by either mini-osmotic pumps or single bolus i.p. Conversely, induction of Th1 cell unresponsiveness by soluble protein is observed in any mouse strain tested, following any mode of Ag administration. These data demonstrate that non-MHC-linked genetic polymorphism controls the priming of Th2 but not the inhibition of Th1 cells induced by administration of soluble protein. The pattern of Th2 responses in these different strains is predictive of disease outcome following Leishmania major infection and supports the hypothesis that systemic Ag presentation in the absence of strong inflammatory signals may represent an important stimulus leading to selective Th2 cell development in susceptible mouse strains.

Normal B cells fail to secrete interleukin-12

Interleukin-12 is a key regulatory cytokine produced by antigen-presenting cells (APC) which drives the development of interferon-gamma (IFN-gamma)-producing cells and promotes cell-mediated immunity. Following subcutaneous immunization with protein antigen in adjuvant, dendritic cells (DC) but not small nor large B cells in immune lymph nodes express antigenic complexes and secrete substantial amounts of bioactive IL-12 p75 upon antigen-specific interaction with T cells. We have analyzed secretion of IL-12 p40 and p75 by cell populations enriched in DC, macrophages or B cells in response to nonspecific stimulation or to interaction with antigen-specific CD4+ cells. These APC populations do not produce IL-12 constitutively but, upon stimulation with heat-fixed Staphylococcus aureus and IFN-gamma, IL-12 p40 and p75 are secreted by DC and macrophages, whereas B cells fail to produce IL-12. B cells also fail to secrete IL-12 in response to stimulation with LPS and IFN-gamma. Co-culture with CD4+ T hybridoma cells and antigen induces IL-12 secretion by DC. Up-regulation of IL-12 secretion by interaction with antigen-specific CD4+ T cells is abrogated by anti-class II monoclonal antibodies (mAb), by soluble CD40 molecules and by anti-CD40 ligand mAb, demonstrating a positive feedback between T cells and DC mediated by TCR-peptide/class II and by CD40-CD40 ligand interactions. Expression of class II and CD40 molecules is comparable in B cells and DC, and both APC types activate CD4+ T cells. Yet, even upon interaction with antigen-specific T cells, B cells fail to secrete IL-12. The capacity of B cells to present antigen but not to secrete IL-12 may explain their propensity to selectively drive T helper type 2 cell development.

The mode of protein antigen administration determines preferential presentation of peptide-class II complexes by lymph node dendritic or B cells

We have compared the capacity of dendritic cells (DC) and B cells to present peptide-class II complexes following administration of protein in adjuvant or in soluble form. Three different antigen-presenting cell (APC) populations were separated from draining lymph node cells from mice immunized s.c. with hen egg-white lysozyme (HEL) in adjuvant or with adjuvant only followed by soluble HEL: DC (N418+, class II+, B220-, low buoyant density), large B cells (B220+, low buoyant density) and small B cells (B220+, high buoyant density). HEL peptide-class II complexes displayed by these APC were evaluated by their capacity to activate HEL-specific T hybridoma cells. Following immunization with HEL in adjuvant, DC are the only lymph node APC population expressing detectable HEL peptide-class II complexes. Conversely, after i.v. administration of soluble HEL in mice previously injected with adjuvant only, lymph node B cells are much more efficient than DC in presenting peptide-class II complexes to T cells. Therefore, different modes of protein antigen administration lead to selective expression of antigenic complexes by different APC populations. These data correlate with the observation that, unlike B cells, DC recruited in lymph nodes of mice injected with adjuvant only present in vitro processed protein antigen much less efficiently than synthetic peptides, probably as a consequence of their maturation in vivo.

Dendritic cells but not B cells present antigenic complexes to class II-restricted T cells after administration of protein in adjuvant

We have analyzed the relative contribution of dendritic cells (DC) and B cells in the presentation of peptide-class II complexes in an inflammatory situation in vivo. Draining lymph node cells from mice immunized subcutaneously with hen egg-white lysozyme (HEL) in adjuvant display HEL peptide-major histocompatibility complex class II complexes able to stimulate, in the absence of any further antigen addition, specific T hybridoma cells. The antigen-presenting capacity of three different antigen-presenting cell (APC) populations recruited in lymph nodes, DC (N418+, class II+, B220-, low buoyant density), large B cells (B220+, low buoyant density), and small B cells (B220+, high buoyant density), was analyzed. After immunization with HEL in adjuvant, DC are the only lymph node APC population expressing detectable HEL peptide-class II complexes. These results indicate that lymph node DC and not B cells are the APC initiating the immune response in vivo after administration of antigen in adjuvant.

Manipulation of the Th1/Th2 cell balance: an approach to treat human autoimmune diseases?

Differentiated T cells produce a restricted set of lymphokines, allowing their subdivision into two major subsets: Th1 and Th2 cells. This has lead to a new paradigm for immunoregulation based on the Th1/Th2 dichotomy. A strict compartmentalization of T cells into Th1 and Th2 is clearly an oversimplification: regulatory and effector mechanisms in the immune system encompass much more than Th1 and Th2 cells. This oversimplification is nevertheless useful to carry out experiments designed to test the paradigm. Based on results obtained in different experimental models of autoimmune diseases, the subdivision of T cells into Th1 and Th2 subsets has been extended to suggest that Th1 cells contribute to the pathogenesis of several organ-specific autoimmune diseases, whereas Th2 cells may inhibit disease development. Although more slowly and maybe less clearly, a similar dichotomy is starting to emerge in human autoimmune diseases. It will soon be possible to formally test immunointervention based on Th1/Th2 cell manipulation in clinical situations: the tools and a conceptual frame are already available. In this review we will examine two key factors affecting the Th1/Th2 balance: antigen and the role of cytokines influencing the development of Th1 and Th2 cells. The rational manipulation of these two variables may ultimately lead to an effective control of Th1 and Th2 cells potentially able to alter the natural course of human autoimmune diseases.

DR alpha: E beta heterodimers in DRA transgenic mice hinder expression of E alpha: E beta molecules and are more efficient in antigen presentation

HLA-DRA transgenic (tg) mice on H-2d background were constructed to study assembly, expression and function of DR alpha: E beta class II heterodimers when an alternate E alpha chain is available. Cytofluorimetric analysis and immunoprecipitation studies demonstrate that the majority (90%) of E beta d molecules on class II-positive splenocytes from DRA-tg mice are associated with DR alpha rather than E alpha chains. To characterize the functional role of the interspecies as compared with the wild-type I-E molecules, MHC restriction and T cell epitope immunodominance of synthetic peptides spanning the entire sequence of 65 kDa heat shock protein (hsp) from Mycobacterium tuberculosis were determined in hsp-primed DRA-tg and DBA/2 mice. A similar pattern of responsiveness was observed in both strains, but hsp epitopes recalled a higher response in DRA-tg as compared with DBA/2 mice. A panel of T cell hybridomas specific for two hsp peptides or a hen egg white lysozyme peptide presented by both DR alpha: E beta d and E alpha d: E beta d was studied in detail. Surprisingly, DR alpha: E beta d dimers present these peptides more efficiently than E alpha d: E beta d, even when the TCR was selected in mice expressing only E alpha d: E beta d molecules. The higher efficiency of antigen presentation by DR alpha: E beta d dimers does not appear to depend on increased binding affinity for peptides, as demonstrated by competition for antigen presentation, nor on increased efficiency in the interaction with CD4 molecules. Rather, the higher efficiency of antigen presentation could be explained by a more effective ligand-TCR interaction. This is consistent with molecular modeling based on the class II structure, indicating that 16 out of 17 substitutions between the first domain of E alpha d and DR alpha chains ile outside the peptide binding groove and are potentially available for interaction with the TCR.

Constitutive presentation of dominant epitopes from endogenous naturally processed self-beta 2-microglobulin to class II-restricted T cells leads to self-tolerance

The mouse beta 2-microglobulin (m beta 2-m) peptide corresponding to residues 25-40 binds to the MHC class II molecules I-Ad and I-Ed and is immunogenic in BALB/c beta 2-m-deficient but not in normal BALB/c mice. The self-m beta 2-m peptide 25-40 is presented by both I-Ad and I-Ed class II molecules as demonstrated by the activation of T cell hybridomas specific for this sequence obtained from beta 2-m knock-out mice. By analyzing the effect of N- and C-terminal truncations of m beta 2-m25-40 on binding to class II molecules and on activation of T cell hybridomas, the minimum epitopes recognized by I-Ad and I-Ed-restricted T cells are included within amino acid residues 26-39 and 24-36, respectively. Both sets of T hybridomas are also activated by the corresponding naturally processed self-epitope presented by APC from BALB/c mice and from other H-2d strains, irrespective of their Mls phenotype. Therefore, the sequence 25-40 contains dominant naturally processed self-epitopes of the mouse beta 2-m. Processing of endogenous m beta 2-m is sensitive to protease inhibitors and lysosomotropic amines, and is not caused by reuptake of shed or released protein. These results indicate that self-beta 2-m-peptide-MHC class II complexes derive from constitutive processing of the endogenous intracellular pool of m beta 2-m in an acidic endosomal compartment. Antigenic complexes between m beta 2-m peptides and I-Ad or I-Ed class II molecules are constitutively expressed by APC of different tissues, including the thymus, and they are able to induce T cell tolerance, as shown by the lack of T cell response to m beta 2-m25-40 in BALB/c mice.

Dendritic cells are the most efficient in presenting endogenous naturally processed self-epitopes to class II-restricted T cells

Dendritic cells (DC) are potent APCs, able to induce efficiently primary T cell-mediated responses to foreign Ags. To assess the efficiency of DC, as compared with other APC types, in the in vivo presentation of self-Ags to CD4+ T cells, we analyzed processing and presentation to class II-restricted T cells of endogenous naturally processed self-epitopes constitutively expressed by mouse APC. Mouse beta 2-microglobulin (m beta 2-m) peptides corresponding to residues 26-39 and 24-36 are constitutively presented, in mice expressing m beta 2-m, by I-Ad and I-Ed molecules respectively, as demonstrated by activation of m beta 2-m-specific T cell hybridomas generated in BALB/c beta 2-m-deficient mice. These dominant, naturally processed self-epitopes of m beta 2-m are presented by APC from a variety of tissues, including the thymus. To analyze the relative efficiency of different APC populations in the presentation of self-beta 2-m, the ability of purified DC, macrophages, and large or small B cells to stimulate m beta 2-m-specific T cell hybridomas was tested. Naturally processed self-m beta 2-m epitopes are constitutively presented to T cells by any class II-positive APC tested, but with highest efficiency by splenic and thymic DC, followed by macrophages, large B cells, and small B cells. This hierarchy of self-beta 2-m presentation does not depend on differential processing capacity of these APC populations, and it correlates with expression of CTLA-4 ligands and ICAM-1 molecules, rather than with expression of class II molecules.

Constitutive presentation of dominant epitopes from endogenous naturally processed self-beta 2-microglobulin to class II-restricted T cells leads to self-tolerance

The mouse beta 2-microglobulin (m beta 2-m) peptide corresponding to residues 25-40 binds to the MHC class II molecules I-Ad and I-Ed and is immunogenic in BALB/c beta 2-m-deficient but not in normal BALB/c mice. The self-m beta 2-m peptide 25-40 is presented by both I-Ad and I-Ed class II molecules as demonstrated by the activation of T cell hybridomas specific for this sequence obtained from beta 2-m knock-out mice. By analyzing the effect of N- and C-terminal truncations of m beta 2-m25-40 on binding to class II molecules and on activation of T cell hybridomas, the minimum epitopes recognized by I-Ad and I-Ed-restricted T cells are included within amino acid residues 26-39 and 24-36, respectively. Both sets of T hybridomas are also activated by the corresponding naturally processed self-epitope presented by APC from BALB/c mice and from other H-2d strains, irrespective of their Mls phenotype. Therefore, the sequence 25-40 contains dominant naturally processed self-epitopes of the mouse beta 2-m. Processing of endogenous m beta 2-m is sensitive to protease inhibitors and lysosomotropic amines, and is not caused by reuptake of shed or released protein. These results indicate that self-beta 2-m-peptide-MHC class II complexes derive from constitutive processing of the endogenous intracellular pool of m beta 2-m in an acidic endosomal compartment. Antigenic complexes between m beta 2-m peptides and I-Ad or I-Ed class II molecules are constitutively expressed by APC of different tissues, including the thymus, and they are able to induce T cell tolerance, as shown by the lack of T cell response to m beta 2-m25-40 in BALB/c mice.

Dendritic cells are the most efficient in presenting endogenous naturally processed self-epitopes to class II-restricted T cells

Dendritic cells (DC) are potent APCs, able to induce efficiently primary T cell-mediated responses to foreign Ags. To assess the efficiency of DC, as compared with other APC types, in the in vivo presentation of self-Ags to CD4+ T cells, we analyzed processing and presentation to class II-restricted T cells of endogenous naturally processed self-epitopes constitutively expressed by mouse APC. Mouse beta 2-microglobulin (m beta 2-m) peptides corresponding to residues 26-39 and 24-36 are constitutively presented, in mice expressing m beta 2-m, by I-Ad and I-Ed molecules respectively, as demonstrated by activation of m beta 2-m-specific T cell hybridomas generated in BALB/c beta 2-m-deficient mice. These dominant, naturally processed self-epitopes of m beta 2-m are presented by APC from a variety of tissues, including the thymus. To analyze the relative efficiency of different APC populations in the presentation of self-beta 2-m, the ability of purified DC, macrophages, and large or small B cells to stimulate m beta 2-m-specific T cell hybridomas was tested. Naturally processed self-m beta 2-m epitopes are constitutively presented to T cells by any class II-positive APC tested, but with highest efficiency by splenic and thymic DC, followed by macrophages, large B cells, and small B cells. This hierarchy of self-beta 2-m presentation does not depend on differential processing capacity of these APC populations, and it correlates with expression of CTLA-4 ligands and ICAM-1 molecules, rather than with expression of class II molecules.

Processing of endogenously synthesized hen egg-white lysozyme retained in the endoplasmic reticulum or in secretory form gives rise to a similar but not identical set of epitopes recognized by class II-restricted T cells

To study the processing and presentation of endogenously synthesized Ag to class II MHC-restricted T cells, hen egg lysozyme (HEL), either tagged with a peptide that confers retention in the endoplasmic reticulum (HEL.KDEL), or in the secretory form (HELs), was stably expressed in LK-35.2 B hybridoma cells. Presentation of HEL peptides bound to class II molecules was assessed by activation of specific T cell hybridomas recognizing seven different epitopes derived from exogenous HEL. The presentation of endogenously synthesized HEL was not caused by reuptake of secreted of shed Ag. All the HEL epitopes examined were efficiently presented after processing of endogenous HEL by HELs-transfected LK-35.2 cells. Processing of HEL tagged with KDEL, however, gave rise to presentation of only six of the seven HEL epitopes. The epitope included in the HEL sequence 112-124 was not presented by HEL.KDEL-transfected B cells. In addition, two of the four T cell hybridomas recognizing HEL 116-129 together with I-Ak molecules were not activated by HEL.KDEL, and three other epitopes were presented with lower efficiency as compared with HELs. Thus, endogenously synthesized HEL in secretory form gives rise to a set of class II-binding epitopes indistinguishable from exogenous HEL, whereas endoplasmic reticulum-retained HEL generates a similar but not identical set of epitopes. The endosomal protease inhibitor leupeptin prevented presentation of the epitope 108-116, but not 46-61, both by HELs and HEL.KDEL transfected cells, indicating a requirement for endosomal processing in both cases. In addition, the presentation of peptides derived from endogenously synthesized, either secretory or endoplasmic reticulum-retained HEL, could be inhibited by lysosomotropic amines, further indicating that the intracellular route of class II molecules presenting peptides derived from endogenous Ag intersects the acidic endosomal compartment.

Processing of endogenously synthesized hen egg-white lysozyme retained in the endoplasmic reticulum or in secretory form gives rise to a similar but not identical set of epitopes recognized by class II-restricted T cells

To study the processing and presentation of endogenously synthesized Ag to class II MHC-restricted T cells, hen egg lysozyme (HEL), either tagged with a peptide that confers retention in the endoplasmic reticulum (HEL.KDEL), or in the secretory form (HELs), was stably expressed in LK-35.2 B hybridoma cells. Presentation of HEL peptides bound to class II molecules was assessed by activation of specific T cell hybridomas recognizing seven different epitopes derived from exogenous HEL. The presentation of endogenously synthesized HEL was not caused by reuptake of secreted of shed Ag. All the HEL epitopes examined were efficiently presented after processing of endogenous HEL by HELs-transfected LK-35.2 cells. Processing of HEL tagged with KDEL, however, gave rise to presentation of only six of the seven HEL epitopes. The epitope included in the HEL sequence 112-124 was not presented by HEL.KDEL-transfected B cells. In addition, two of the four T cell hybridomas recognizing HEL 116-129 together with I-Ak molecules were not activated by HEL.KDEL, and three other epitopes were presented with lower efficiency as compared with HELs. Thus, endogenously synthesized HEL in secretory form gives rise to a set of class II-binding epitopes indistinguishable from exogenous HEL, whereas endoplasmic reticulum-retained HEL generates a similar but not identical set of epitopes. The endosomal protease inhibitor leupeptin prevented presentation of the epitope 108-116, but not 46-61, both by HELs and HEL.KDEL transfected cells, indicating a requirement for endosomal processing in both cases. In addition, the presentation of peptides derived from endogenously synthesized, either secretory or endoplasmic reticulum-retained HEL, could be inhibited by lysosomotropic amines, further indicating that the intracellular route of class II molecules presenting peptides derived from endogenous Ag intersects the acidic endosomal compartment.

Selective immunosuppression

Experimental models of autoimmune diseases have demonstrated that such disease can be prevented or treated by selectively interfering with activation of any of these cell types: antigen-presenting cells, autoreactive T cells and regulatory T cells. Luciano Adorini and colleagues discuss these approaches to selective immunosuppression and examine how similar strategies may become applicable to the treatment of human autoimmune diseases.

MHC class II molecules bind indiscriminately self and non-self peptide homologs: effect on the immunogenicity of non-self peptides

Synthetic peptides spanning the entire sequence of both human and mouse beta 2-microglobulin (beta 2M) have been tested for their capacity to bind to three different mouse (I-Ad, I-Ed, and I-Ak) or human (DR1, DR2, and DR5) class II molecules. The results demonstrate that class II molecules do not discriminate between self and non-self peptides. When the immunogenicity of the human beta 2M peptides was measured by their ability to prime H-2d mice for in vitro T cell proliferation, it was found that peptides incapable of binding class II molecules in vitro were also non-immunogenic in vivo. Interestingly, however, several binders, including the human beta 2M peptide 1-16, the best binder in this series to Iad molecules, were found to be non-immunogenic. Since the corresponding mouse beta 2M peptide 1-16 was also capable of binding to Iad molecules, this suggested that lack of responsiveness to the non-self peptide could arise either from central or peripheral tolerance induced by the self homolog. Alternatively, lack of responsiveness could arise from other mechanisms, such as negative selection by other non-homolog sequences or lack of suitable T cell receptor genes. To discriminate between these possibilities, H-2d mice with disrupted beta 2M genes were immunized with the human beta 2M peptide 1-16. This peptide also failed to prime for T cell responsiveness in beta 2M-negative mice, suggesting that a hole in the T cell repertoire for this antigen was not mediated by negative selection or peripheral tolerance induced by self beta 2M peptides.

Selective immunosuppression

Experimental models of autoimmune diseases have demonstrated that such disease can be prevented or treated by selectively interfering with activation of any of these cell types: antigen-presenting cells, autoreactive T cells and regulatory T cells. Luciano Adorini and colleagues discuss these approaches to selective immunosuppression and examine how similar strategies may become applicable to the treatment of human autoimmune diseases.

Selective immunosuppression by administration of major histocompatibility complex class II-binding peptides. II. Preventive inhibition of primary and secondary in vivo antibody responses

The self-mouse lysozyme peptide corresponding to residues 46-62 (ML46-62) binds to the major histocompatibility complex (MHC) class II molecules I-A(k) and it selectively inhibits, when coinjected with antigen, priming of I-A(k)-restricted, antigen-specific T cells. We demonstrate that administration of ML46-62 also inhibits in vivo antibody responses induced by I-A(k)-restricted T helper cells. ML46-62 is able to prevent the primary anti-hen egg white lysozyme (HEL) antibody response induced by the entire HEL molecule in B10.A(4R) mice, expressing only I-A(k) molecules, but not in mice of H-2d haplotype. ML46-62 also strongly decreases, in B10.A(4R) mice, the antibody response to ribonuclease A, a protein antigen unrelated to the MHC blocker, indicating that MHC blockade is the mechanism leading to inhibition of antibody response. This is further supported by the concomitant decrease, in vivo, of complex formation between immunodominant HEL peptides and I-A(k) molecules, preventing I-A(k)-restricted T cell induction. Administration of ML46-62 after antigen priming does not affect ongoing antibody responses, as expected from MHC blockade. A single injection of ML46-62 at the time of protein antigen priming precludes not only the primary, but also the secondary antibody response to a subsequent challenge with soluble protein, even when the challenge is performed several months after priming. Coinjection of antigen and MHC antagonist inhibits production of all antibody isotypes equally well, suggesting that MHC class II blockade affects both Th1- and Th2-type T helper cells. Therefore, these results indicate that administration of MHC class II-binding peptides can efficiently and selectively prevent the induction of T cell-dependent primary and secondary in vivo antibody responses by blocking antigen presentation to class II-restricted T helper cells.

Selective immunosuppression by administration of major histocompatibility complex (MHC) class II-binding peptides. I. Evidence for in vivo MHC blockade preventing T cell activation

Draining lymph node cells (LNC) from mice immunized with hen egg white lysozyme (HEL) display at their surface antigen-MHC complexes able to stimulate, in the absence of any further antigen addition, HEL peptide-specific, class II-restricted T cell hybridomas. Chloroquine addition to these LNC cultures fails to inhibit antigen presentation, indicating that antigenic complexes of class II molecules and HEL peptides are formed in vivo. MHC class II restriction of antigen presentation by LNC from HEL-primed mice was verified by the use of anti-class II monoclonal antibodies. Coinjection of HEL and the I-Ak-binding peptide HEL 112-129 in mice of H-2k haplotype inhibits the ability of LNC to stimulate I-Ak-restricted, HEL 46-61-specific T cell hybridomas. Similar results are obtained in mice coinjected with the HEL peptides 46-61 and 112-129. Inhibition of T hybridoma activation can also be observed using as antigen-presenting cells irradiated, T cell-depleted LNC from mice coinjected with HEL 46-61 and HEL 112-129, ruling out the possible role of either specific or nonspecific suppressor T cells. Inhibition of T cell proliferation is associated with MHC-specific inhibition of antigen presentation and with occupancy by the competitor of class II binding sites, as measured by activation of peptide-specific T cell hybridomas. These results demonstrate that administration of MHC class II binding peptide competitors selectively inhibits antigen presentation to class II-restricted T cells, indicating competitive blockade of class II molecules in vivo.

Exogenous peptides compete for the presentation of endogenous antigens to major histocompatibility complex class II-restricted T cells

Antigen-presenting cells (APC) transfected with a construct encoding the hen egg-white lysozyme (HEL) amino acid sequence 1-80 constitutively present HEL peptides complexed to major histocompatibility complex (MHC) class II molecules to specific T cell hybridomas, indicating that endogenous cellular antigens can be efficiently presented to class II-restricted T cells. Here we show that exogenous peptide competitors added to HEL-transfected APC can inhibit the presentation of endogenous HEL peptides to class II-restricted T cells. The inhibition is specific for the class II molecule binding the competitor peptide, and it affects to the same extent presentation of exogenous or endogenous HEL peptides. These results, demonstrating that an exogenous competitor can inhibit class II-restricted T cell activation induced by endogenous as well as exogenous antigen, suggest lack of strict compartmentalization between endogenous and exogenous pathways of antigen presentation. Since autoreactive T cells may recognize endogenous, as well as exogenous antigens, the results have implications for the treatment of autoimmune diseases by MHC blockade.

Inhibition of T cell activation by blockade of MHC class II molecules

Autoimmune diseases result from the activation of self-reactive T cells induced by autoantigens or by foreign antigens cross-reactive with an autoantigen. A striking characteristic of autoimmune diseases is the increased frequency of certain HLA alleles in affected individuals. Moreover, as demonstrated for example in rheumatoid arthritis and insulin-dependent diabetes mellitus, class II alleles positively associated with autoimmune diseases share amino acid residues in the hypervariable HLA regions involved in peptide binding. Therefore, it is likely that disease-associated HLA class II molecules have the capacity to bind the autoantigen and present it to T cells, thereby inducing and maintaining, under appropriate conditions, the autoimmune disease. The data reviewed here demonstrate MHC-selective inhibition of antigen-induced T cell responses in vivo by parenterally administered soluble, MHC-binding peptide competitors, under conditions in which the competitor is not immunogenic. This suggests the feasibility of a therapeutic approach based on MHC blockade in the treatment of HLA-linked autoimmune diseases.

A spontaneous hybridoma producing autoanti-idiotypic antibodies that recognize a V kappa-associated idiotope in mercury-induced autoimmunity

Anti-idiotypic (Id) antibodies have been suggested to play a role in the self regulation process observed in Brown-Norway rats developing mercury-induced autoimmunity. However, the presence of such antibodies has not yet been directly demonstrated. For that purpose, spleen cells from a mercury-injected rat were fused and the resulting hybridomas tested for their anti-Id activity against monoclonal anti-glomerular basement membrane (GBM) antibodies produced in this model. A monoclonal antibody (mAb) was obtained that specifically reacted in an enzyme-linked immunosorbent assay with an anti-GBM mAb and to a much lesser extent with another one produced in the same fusion. In Western blot experiments this autoanti-Id mAb reacted under reducing conditions with the kappa L chains but not with the H chains of the two anti-GBM mAb. It did not react with the kappa L chains of eight other rat mAb. This mAb is therefore an autoanti-Id mAb that recognizes a V kappa-associated Id expressed on two anti-GBM mAb. These results demonstrate that anti-GBM antibodies and their corresponding autoanti-Id antibodies are simultaneously produced during this disease. Whether or not these autoanti-Id antibodies have a regulatory and/or a pathogenic role in this disease remains to be established.

Rat anti-glomerular basement membrane antibodies in toxin-induced autoimmunity and in chronic graft-vs.-host reaction share recurrent idiotypes

Cross-reactive idiotypes (CRId) borne on autoanti-glomerular basement membrane antibodies of Brown-Norway (BN) rats with mercury-induced glomerulonephritis have been described in the preceding study (Guéry, J.-C. et al., Eur. J. Immunol. 1990. 20:93). BN rats treated with sodium aurothiopropanol sulfonate or D-penicillamine, as well as (LEW X BN)F1 hybrids transferred with BN rat spleen cells, developed quite similar autoimmune abnormalities. In the present study, it is shown that immunoglobulins bearing such "public" idiotypes are also produced and deposited in the kidney in these three models. The CRId here described may, therefore, be considered as a marker of sets of recurrently expressed V region genes during the course of these autoimmune disorders. Anti-self class II T cells are present in the three models of toxin-induced autoimmunity and anti-allo class II T cells are responsible for the chronic graft-vs.-host reaction. The same B cell clones are probably triggered during these processes as a consequence of a polyclonal B cell activation mediated by anti-class II T cells.

Specificity and cross-reactive idiotypes of anti-glomerular basement membrane autoantibodies in HgCl2-induced autoimmune glomerulonephritis

Mercury-induced autoimmune glomerulonephritis in the Brown-Norway (BN) rat is characterized by the successive appearance of linear and granular glomerular IgG deposits. Anti-laminin autoantibodies represent the major part of the anti-glomerular basement membrane (GBM) antibodies produced in this model. Fusions were performed in this model and four anti-GBM monoclonal antibodies (mAb) were obtained. Three of them were laminin specific. Using rabbit anti-idiotype antibodies, cross-reactive idiotypes (CRId) were characterized on anti-laminin antibodies. They were expressed on the three anti-laminin mAb, on kidney-eluted and circulating anti-laminin antibodies. CRId-bearing immunoglobulins were detected transiently in the circulation and paralleled the anti-laminin antibody activity. By immunofluorescence studies on kidney cryostat sections two different CRId were defined. One was localized close to the antigen-combining site since it was not revealed on kidney-bound antibodies, in contrast with the second CRId. This latter CRId was also found deposited in a typical linear pattern in the early phase of the disease and in a granular pattern in the late phase, demonstrating that these CRId are components of immune deposits. Taken together, these results suggest that in this model of T-dependent polyclonal B cell activation, restricted sets of V genes encode for at least a part of the anti-GBM autoantibodies.