Insulin-specific T cell hybridomas derived from (H-2b x H-2k)F1 mice preferably employ F1-unique restriction elements for antigen recognition

The lymphoid lineage-specific actin-uncapping protein Rltpr is essential for costimulation via CD28 and the development of regulatory T cells

Although T cell activation can result from signaling via T cell antigen receptor (TCR) alone, physiological T cell responses require costimulation via the coreceptor CD28. Through the use of an N-ethyl-N-nitrosourea-mutagenesis screen, we identified a mutation in Rltpr. We found that Rltpr was a lymphoid cell-specific, actin-uncapping protein essential for costimulation via CD28 and the development of regulatory T cells. Engagement of TCR-CD28 at the immunological synapse resulted in the colocalization of CD28 with both wild-type and mutant Rltpr proteins. However, the connection between CD28 and protein kinase C-θ and Carma1, two key effectors of CD28 costimulation, was abrogated in T cells expressing mutant Rltpr, and CD28 costimulation did not occur in those cells. Our findings provide a more complete model of CD28 costimulation in which Rltpr has a key role.

Lck-dependent Fyn activation requires C terminus-dependent targeting of kinase-active Lck to lipid rafts

Mechanisms regulating the activation and delivery of function of Lck and Fyn are central to the generation of the most proximal signaling events emanating from the T cell antigen receptor (TcR) complex. Recent results demonstrate that lipid rafts (LR) segregate Lck and Fyn and play a fundamental role in the temporal and spatial coordination of their activation. Specifically, TcR-CD4 co-aggregation-induced Lck activation outside LR results in Lck translocation to LR where the activation of LR-resident Fyn ensues. Here we report a structure-function analysis toward characterizing the mechanism supporting Lck partitioning to LR and its capacity to activate co-localized Fyn. Using NIH 3T3 cells ectopically expressing FynT, we demonstrate that only LR-associated, kinase-active (Y505F)Lck reciprocally co-immunoprecipitates with and activates Fyn. Mutational analyses revealed a profound reduction in the formation of Lck-Fyn complexes and Fyn activation, using kinase domain mutants K273R and Y394F of (Y505F)Lck, both of which have profoundly compromised kinase activity. The only kinase-active Lck mutants tested that revealed impaired physical and enzymatic engagement with Fyn were those involving truncation of the C-terminal sequence YQPQP. Remarkably, sequential truncation of YQPQP resulted in an increasing reduction of kinase-active Lck partitioning to LR, in both fibroblasts and T cells. This in turn correlated with an ablation of the capacity of these truncates to enhance TcR-mediated interleukin-2 production. Thus, Lck-dependent Fyn activation is predicated by proximity-mediated transphosphorylation of the Fyn kinase domain, and targeting kinase-active Lck to LR is dependent on the C-terminal sequence QPQP.

Mixed-haplotype MHC class II molecules select functional CD4+ T cells

MHC class II molecules are formed from polymorphic alpha and beta chains. While pairing of chains is most efficient within class II isotypes and haplotypes, limited pairing and surface expression of mixed-haplotype and -isotype class II molecules is common. The function of such molecules in antigen presentation has been established by the unique restriction of responses in F1 mice. However, it has not been established whether mixed class II molecules are able to mediate selection of functional T cells and how the reduced avidity of the TCR/MHC interaction influences the repertoire. In this report we have addressed these issues through the production of mice expressing solely mixed-haplotype class II molecules. The mixed class II molecules promote selection of a small CD4+ T cell repertoire with modified TCR use. The selected CD4+ T cells are functional in vivo and in vitro.

Molecular basis for the high affinity interaction between the thymic leukemia antigen and the CD8alphaalpha molecule

The mouse thymic leukemia (TL) Ag is a nonclassical MHC class I molecule that binds with higher affinity to CD8alphaalpha than CD8alphabeta. The interaction of CD8alphaalpha with TL is important for lymphocyte regulation in the intestine. Therefore, we studied the molecular basis for TL Ag binding to CD8alphaalpha. The stronger affinity of the TL Ag for CD8alphaalpha is largely mediated by three amino acids on exposed loops of the conserved alpha3 domain. Mutant classical class I molecules substituted with TL Ag amino acids at these positions mimic the ability to interact with CD8alphaalpha and modulate lymphocyte function. These data indicate that small changes in the alpha3 domain of class I molecules potentially can have profound physiologic consequences.

Regulation of SOCS-1 expression by translational repression

Accumulating evidence demonstrates that cytokine receptor signaling is negatively regulated by a family of Src homology 2 domain-containing adaptor molecules termed SOCS (suppressor of cytokine signaling). Previous studies have indicated that the expression of SOCS-related molecules is tightly controlled at the level of transcription. Furthermore, it has been reported that SOCS polypeptides are relatively unstable in cells, unless they are associated with elongins B and C. Herein, we document the existence of a third mechanism of regulation of SOCS function. Our data showed that expression of SOCS-1, a member of the SOCS family, is strongly repressed at the level of translation initiation. Structure-function analyses indicated that this effect is mediated by the 5' untranslated region of socs-1 and that it relates to the presence of two upstream AUGs in this region. Further studies revealed that socs-1 translation is cap-dependent and that it is modulated by eIF4E-binding proteins. In combination, these results uncover a novel level of regulation of SOCS-related molecules. Moreover, coupled with previous findings, they suggest that SOCS expression is tightly regulated through multiple mechanisms, in order to avoid inappropriate interference with cytokine-mediated effects.

Recessive expression of the H2A-controlled immune response phenotype depends critically on antigen dose

Major histocompatibility complex (MHC) alleles acting as immune response genes are coexpressed in heterozygous individuals and therefore control of immune responses is usually codominant. As an exception to this rule, however, several examples of recessive immune responses have been ascribed to regulatory, e.g. suppressive, interactions. We report here that the recessive phenotype of both antibody and T-cell responses to the mycobacterial 16 000-MW antigen depends critically on a low antigen dose for immunization. On the basis of similar responses in hemi- and heterozygous mice, we suggest that the mechanism of recessive MHC control does not involve regulation by the low-responder allele. We also demonstrated mixed haplotype restriction of peptide recognition for a significant fraction of high-antigen-dose primed T cells. Their paucity under limiting antigen dose conditions may lead to the recessive expression of MHC control. In conclusion, our results suggest that recessive MHC control can be explained as a simple gene dosage effect under conditions where antigen is limiting, without a need for regulatory mechanisms.

Interactions of CD45-associated protein with the antigen receptor signaling machinery in T-lymphocytes

CD45 is a transmembrane protein tyrosine phosphatase playing an essential role during T-cell activation. This function relates to the ability of CD45 to regulate p56(lck), a cytoplasmic protein tyrosine kinase necessary for T-cell antigen receptor (TCR) signaling. Previous studies have demonstrated that CD45 is constitutively associated in T-lymphocytes with a transmembrane molecule termed CD45-AP (or lymphocyte phosphatase-associated phosphoprotein). Even though the exact role of this polypeptide is unclear, recent analyses of mice lacking CD45-AP have indicated that its expression is also required for optimal T-cell activation. Herein, we wished to understand better the function of CD45-AP. The results of our studies showed that in T-cells, CD45-AP is part of a multimolecular complex that includes not only CD45, but also TCR, the CD4 and CD8 coreceptors, and p56(lck). The association of CD45-AP with TCR, CD4, and CD8 seemed to occur via the shared ability of these molecules to bind CD45. However, binding of CD45-AP to p56(lck) could take place in the absence of other lymphoid-specific components, suggesting that it can be direct. Structure-function analyses demonstrated that such an interaction was mediated by an acidic segment in the cytoplasmic region of CD45-AP and by the kinase domain of p56(lck). Interestingly, the ability of CD45-AP to interact with Lck in the absence of other lymphoid-specific molecules was proportional to the degree of catalytic activation of p56(lck). Together, these findings suggest that CD45-AP is an adaptor molecule involved in orchestrating interactions among components of the antigen receptor signaling machinery. Moreover, they raise the possibility that one of the functions of CD45-AP is to recognize activated Lck molecules and bring them into the vicinity of CD45.

Cooperative inhibition of T-cell antigen receptor signaling by a complex between a kinase and a phosphatase

Antigen receptor-triggered T-cell activation is mediated by the sequential action of the Src and Syk/Zap-70 families of protein tyrosine kinases (PTKs). Previously, we reported that another PTK termed p50(csk) was a potent negative regulator of T-cell receptor (TCR) signaling because of its ability to inactivate Src-related kinases. This inhibitory effect required the catalytic activity of Csk, as well as its Src homology (SH)3 and SH2 domains. Subsequent studies uncovered that, via its SH3 domain, p50(csk) was associated with PEP, a proline-enriched protein tyrosine phosphatase (PTP) of unknown function expressed in hemopoietic cells. Herein, we have attempted to identify the role of the Csk-PEP complex in T lymphocytes. The results of our experiments showed that, like Csk, PEP was a strong repressor of TCR signaling. This property was dependent on the phosphatase activity of PEP, as well as on the sequence mediating its binding to p50(csk). Through reconstitution experiments in Cos-1 cells, evidence was obtained that Csk and PEP act synergistically to inhibit protein tyrosine phosphorylation by Src-related kinases, and that this effect requires their association. Finally, experiments with a substrate-trapping mutant of PEP suggested that PEP functions by dephosphorylating and inactivating the PTKs responsible for T-cell activation. In addition to giving novel insights into the mechanisms involved in the negative regulation of T-cell activation, these findings indicate that the association of an inhibitory PTK with a PTP constitutes a more efficient means of inhibiting signal transduction by Src family kinases in vivo.

Heterogeneity of the T Cell Response to Immunodominant Determinants Within Hen Eggwhite Lysozyme of Individual Syngeneic Hybrid F1 Mice: Implications for Autoimmunity and Infection

Hybrid F1 mice derived from inbred parental mouse strains are extensively used as animal models of human autoimmune diseases and transplantation. It is generally believed that with regard to immunologic studies, hybrid F1 mice behave in a consistent manner, equivalent to any other inbred mouse strain. In this study, we report that in comparison to inbred parental strains, individual hybrid F1 mice revealed a broad heterogeneity of proliferative response to the immunodominant determinants within hen eggwhite lysozyme (HEL). Of five parental strains tested, individual mice of three strains responding to only a few dominant HEL determinants (B6, BALB/c, and B10.PL) showed quite homogeneous patterns of response, whereas two mouse strains responsive to several determinants of HEL revealed either relative homogeneity (CBA/J mice) or heterogeneity (SJL mice) of response. However, in SJL mice, responses to major, dominant determinants of HEL were quite consistent. On the contrary, regardless of the consistency of response of parental strains, all three of F1 mice {[B6 × BALB/c]F1, [B6 × CBA/J]F1, and [SJL × B10.PL]F1} revealed significantly greater heterogeneity of response, which even involved the major, dominant determinants of HEL. We attribute the above heterogeneity of response to the competitive as well as aleatory nature of the interaction between various factors, including the coexistence of different MHC (parental as well as hybrid MHC) molecules, determinant capture, and the T cell repertoire. These results have important implications for studies on autoimmunity, infection, and vaccine design in human populations, where heterozygosity is the norm rather than the exception.

A unique insert in the linker domain of Syk is necessary for its function in immunoreceptor signalling

Accumulating data indicate that the 'linker' region of Syk, which lies between its tandem Src homology 2 (SH2) domains and kinase region, provides a critical function for the biological activity of Syk. This importance has been ascribed to the presence of tyrosine phosphorylation sites capable of mediating the recruitment of cellular effectors. We and others previously identified an alternatively spliced variant of Syk, termed SykB, which lacks a 23 amino acid sequence in the linker domain. As this 'linker insert' is also not present in the closely related enzyme Zap-70, it seems plausible that Syk possesses this unique sequence for functional reasons. To understand its role better, we have compared the abilities of Syk and SykB to participate in immunoreceptor-triggered signal transduction. The results of our experiments revealed that, unlike Syk, SykB was inefficient at coupling stimulation of FcepsilonRI on basophils or the antigen receptor on T cells to the early and late events of cellular activation. Further studies showed that the functional defect in SykB was not caused by the absence of crucial tyrosine phosphorylation sites, or by a reduced intrinsic kinase activity. Rather, it correlated with the reduced ability of SykB to bind phosphorylated immunoreceptor tyrosine-based activation motifs (ITAMs) in vitro and in vivo. In combination, these results demonstrated that the unique insert in the linker domain of Syk is crucial for its capacity to participate in immunoreceptor signalling. Furthermore, they provided evidence that the linker region can regulate the ability of Syk to bind ITAMs, thus identifying a novel function for this domain.

Inhibitory tyrosine protein kinase p50csk is associated with protein-tyrosine phosphatase PTP-PEST in hemopoietic and non-hemopoietic cells

p50(csk) is a cytosolic tyrosine protein kinase expressed in all cell types. Accumulating data show that it inhibits multiple cellular processes, as a consequence of its ability to repress the enzymatic activity of Src family tyrosine protein kinases. We previously demonstrated that, via its Src homology 3 (SH3) domain, Csk is tightly bound to PEP, a protein-tyrosine phosphatase (PTP) exclusively expressed in hemopoietic cells. In this report, we have tested the possibility that Csk also interacts with PTP-PEST, a ubiquitous PTP sharing structural homology with PEP. Our studies revealed that Csk was associated with PTP-PEST in a variety of cell types, including non-hemopoietic cells. This interaction involved the SH3 region of p50(csk) and a proline-rich region (PPPLPERTPESFVLADM) outside the catalytic region of PTP-PEST. Even though both PTP-PEST and PEP were associated with Csk, significant differences were noted between these two PTPs. PTP-PEST, but not PEP, was also complexed with Shc, an adaptor molecule implicated in the Ras pathway. Moreover, PTP-PEST and PEP were found to accumulate primarily in distinct intracellular compartments in cell fractionation studies. In combination, these findings indicated that, like PEP, PTP-PEST is probably involved in Csk-mediated functions in mammalian cells. Moreover, they suggested that the roles of Csk-PTP-PEST and Csk-PEP are likely to be different.

Regulation of T-cell antigen receptor signalling by Syk tyrosine protein kinase

T-cell antigen receptor (TCR) signalling has been shown to involve two classes of tyrosine protein kinases: the Src-related kinases p56(lck) and p59(fyr), and the Zap-70/Syk family kinases. Lck and FynT are postulated to initiate TCR-triggered signal transduction by phosphorylating the CD3 and zeta subunits of the TCR complex. This modification permits the recruitment of Zap-70 and Syk, which are presumed to amplify the TCR-triggered signal, by phosphorylating additional intracellular proteins. While Zap-70 is expressed in all T cells, Syk is present in thymocytes and mature T-cell populations such as intraepithelial gammadelta T cells and naive alphabeta T cells. To better understand the role of Syk in these cells, its impact on the physiology of an antigen-specific T-cell line was tested. Our results showed that compared to Zap-70 alone, Syk was a strong positive regulator of antigen receptor-induced signals in BI-141 cells. Surprisingly, they indicated that, like Src family kinases, Syk augmented TCR-triggered tyrosine phosphorylation of CD3/zeta. Syk, but not Zap-70 alone, could also stimulate tyrosine phosphorylation of a zeta-bearing chimera in transiently transfected Cos-1 cells. Finally, evidence was provided that Syk has the capacity to directly phosphorylate a zeta-derived peptide in vitro. These findings suggested that Syk may have a unique role in T cells, as a consequence of its ability to efficiently phosphorylate multiple components of the TCR signalling cascade. Furthermore, they raised the possibility that Syk can regulate the initiation of TCR signalling, by promoting phosphorylation of the immunoreceptor tyrosine-based activation motifs of the TCR complex.

Chk, a Csk family tyrosine protein kinase, exhibits Csk-like activity in fibroblasts, but not in an antigen-specific T-cell line

The Csk family of tyrosine protein kinases comprises two members named Csk and Chk. These enzymes phosphorylate the carboxyl-terminal tyrosine of Src-related kinases in vitro, thereby repressing their activity. Csk has been found to be necessary for normal embryonic development, and to be a potent negative regulator of antigen receptor signaling in T-lymphocytes. As the functions of Chk in mammalian cells are not known, we examined its ability to carry out Csk-like functions in vivo. Like p50csk, Chk reduced the elevated phosphotyrosine levels and the augmented activity of Src family kinases in Csk-deficient fibroblasts. Contrary to Csk, however, Chk was inefficient at repressing antigen receptor-induced signals in a T-cell line (BI-141). We also noted that Chk, but not Csk, failed to stably associate with cellular membranes following addition of a membrane targeting signal to its amino terminus. This observation suggested that Chk may contain dominant targeting sequences disallowing its recruitment to cellular membranes. Hence, these data demonstrate that Chk can mediate some, but not all, Csk-related functions in vivo. Moreover, they suggest that the "restricted" function of Chk may relate at least in part to its inability to be recruited to certain cellular locales.

Differential intrinsic enzymatic activity of Syk and Zap-70 protein-tyrosine kinases

Syk and Zap-70 are related protein-tyrosine kinases implicated in antigen and Fc receptor signaling. While Zap-70 is restricted to T-cells and natural killer cells, Syk accumulates in B-cells, mast cells, platelets, and immature T-cells. In addition, we found that an isoform of Syk (SykB), which carries a 23-amino acid deletion in the "linker" region, is prominently expressed in bone marrow. To better understand the relative impact of Syk, SykB, and Zap-70 on signal transduction, we compared their intrinsic enzymatic properties in transiently transfected COS-1 cells and in hemopoietic cells. Using modified versions of these enzymes bearing a common Myc epitope at the amino terminus, we determined that the ability of Syk and SykB to undergo autophosphorylation and to phosphorylate erythrocyte band 3 in immune complex kinase reactions was at least 100-fold greater than that of Zap-70. Similarly, Syk and SykB, but not Zap-70, caused prominent tyrosine phosphorylation of p120(c-)cbl in COS-1 cells. A similar pattern of activity was also noted for endogenous Syk and Zap-70 from hemopoietic cells. To understand the structural basis for these characteristics, we also created and analyzed a series of chimeras between Syk and Zap-70. These studies indicated that the catalytic domain of Syk and Zap-70, but not their SH2 domains, linker region or carboxyl-terminal tail, was responsible for their respective activity. Taken together, these data demonstrated that the intrinsic enzymatic activity of Syk and SykB is superior to that of Zap-70 and that such a distinction relates to structural variations in the catalytic domain.

Requirement of the SH3 and SH2 domains for the inhibitory function of tyrosine protein kinase p50csk in T lymphocytes

Previous studies from our laboratory have shown that the cytosolic tyrosine protein kinase p50csk is involved in the negative regulation of T-cell activation (L.M. L. Chow, M. Fournel, D. Davidson, and A. Veillette, Nature [London] 365:156-160, 1993). This function most probably reflects the ability of Csk to phosphorylate the inhibitory carboxy-terminal tyrosine of p56lck and p59fynT, two Src-related enzymes abundantly expressed in T lymphocytes. Herein, we have attempted to better understand the mechanisms by which Csk participates in the inhibitory phase of T-cell receptor signalling. Our results demonstrated that the Src homology 3 (SH3) and SH2 domains of p50csk are crucial for its negative impact on T-cell receptor-mediated signals. As these two sequences were not essential for phosphorylation of the carboxy-terminal tyrosine of a Src-like product in yeast cells, we postulated that they mediate protein-protein interactions allowing the recruitment of p50csk in the vicinity of activated Lck and/or FynT in T cells. In complementary studies, it was observed that linkage of a constitutive membrane targeting signal to the amino terminus of Csk rescued the deleterious impact of a point mutation in the SH2 domain of p50csk. This observation suggested that the SH2 sequence is in part necessary to translocate p50csk from the cytoplasm to the plasma membrane, where Src-related enzymes are located. Nevertheless, constitutive membrane localization was unable to correct the effect of complete deletion of the SH3 or SH2 sequence, implying that these domains provide additional functions necessary for the biological activity of p50csk.

The alpha 4 integrin chain is a ligand for alpha 4 beta 7 and alpha 4 beta 1

The heterodimeric alpha 4 integrins alpha 4 beta 7 lymphocyte Peyer's patch adhesion molecule ([LPAM]-1) and alpha 4 beta 1 (very late antigen-4) are cell surface adhesion molecules involved in lymphocyte trafficking and lymphocyte-cell and matrix interactions. Known cellular ligands include vascular cell adhesion molecule (VCAM)-1, which binds to alpha 4 beta 1 and alpha 4 beta 7, and the mucosal addressin cell adhesion molecule (MAdCAM)-1, which binds to alpha 4 beta 7. Here we show that the alpha 4 chain of these integrins can itself serve as a ligand. The alpha 4 chain, immunoaffinity purified and immobilized on glass slides, binds thymocytes and T lymphocytes. Binding exhibits divalent cation requirements and temperature sensitivity which are characteristic of integrin-mediated interactions, and is specifically inhibited by anti-alpha 4 integrin antibodies, which exert their effect at the cell surface. Cells expressing exclusively alpha 4 beta 7 (TK-1) or alpha 4 beta 1 (L1-2) both bound avidly, whereas alpha 4-negative cells did not. A soluble 34-kD alpha 4 chain fragment retained binding activity, and it inhibited lymphocyte adhesion to alpha 4 ligands. It has been shown that alpha 4 integrin binding to fibronectin involves an leucine-aspartic acid-valine (LDV) motif in the HepII/IIICS region of fibronectin (CS-1 peptide), and homologous sequences are important in binding to VCAM-1 and MAdCAM-1. Three conserved LDV motifs occur in the extracellular sequence of alpha 4. A synthetic LDV-containing alpha 4-derived oligopeptide supports alpha 4-integrin-dependent lymphocyte adhesion and blocks binding to the 34-kD alpha 4 chain fragment. Our results suggest that alpha 4 beta 7 and alpha 4 beta 1 integrins may be able to bind to the alpha 4 subunit on adjacent cells, providing a novel mechanism for alpha 4 integrin-mediated and activation-regulated lymphocyte interactions during immune responses.

The unique amino-terminal domain of p56lck regulates interactions with tyrosine protein phosphatases in T lymphocytes

The catalytic activity of p56lck is repressed by phosphorylation of a conserved carboxy-terminal tyrosine residue (tyrosine 505). Accumulating data show that this phosphorylation is mediated by the tyrosine protein kinase p50csk and that it is reversed by the transmembrane tyrosine protein phosphatase CD45. Recent studies have indicated that dephosphorylation of tyrosine 505 in resting T cells is necessary for the initiation of antigen-induced T-cell activation. To better understand this phenomenon, we have characterized the factors regulating tyrosine 505 phosphorylation in an antigen-specific T-cell line (BI-141). As is the case for other T-cell lines, Lck molecules from unstimulated BI-141 cells exhibited a pronounced dephosphorylation of the inhibitory carboxyl-terminal tyrosine. This state could be corrected by incubation of cells with the tyrosine protein phosphatase inhibitor pervanadate, suggesting that it reflected the unrestricted action of tyrosine protein phosphatases. In structure-function analyses, mutation of the site of Lck myristylation (glycine 2) partially restored phosphorylation at tyrosine 505 in BI-141 cells. Since the myristylation-defective mutant also failed to stably associate with cellular membranes, this effect was most probably the consequence of removal of p56lck from the vicinity of membrane phosphatases like CD45. Deletion of the unique domain of Lck, or its replacement by the equivalent sequence from p59fyn, also increased the extent of tyrosine 505 phosphorylation in vivo. This effect was unrelated to changes in Lck membrane association and therefore was potentially related to defects in crucial protein-protein interactions at the membrane. In contrast, deletion of the SH3 or SH2 domain, or mutation of the phosphotransfer motif (lysine 273) or the site of autophosphorylation (tyrosine 394), had no impact on phosphate occupancy at tyrosine 505. In combination, these results indicated that the hypophosphorylation of the inhibitory tyrosine of p56(lck) in T lymphocytes is likely the result of the predominant action of tyrosine protein phosphatases. Moreover, they showed that both the amino-terminal myristylation signal and the unique domain of p56(lck) play critical roles in this process.

Regulation of Zap-70 by Src family tyrosine protein kinases in an antigen-specific T-cell line

To further understand the interactions between Zap-70, Src family kinases, and other T-cell proteins, we have examined the regulation of Zap-70 in the antigen-specific T-cell line BI-141. By analyzing derivatives containing an activated version of either p56lck or p59fynT, it was observed that the two Src-related enzymes augmented T-cell receptor (TCR)-mediated tyrosine phosphorylation of Zap-70, as well as its association with components of the antigen receptor complex. Importantly, the accumulation of TCR.Zap-70 complexes quantitatively and temporally correlated with the induction of tyrosine phosphorylation of the CD3 and zeta chains of TCR. Using a CD4-positive variant of BI-141, we also found that the ability of Zap-70 to undergo tyrosine phosphorylation and associate with TCR was enhanced by aggregation of TCR with the CD4 co-receptor. Further studies allowed the identification of two distinct pools of tyrosine-phosphorylated Zap-70 in activated T-cells. While one population was associated with TCR, the other was co-immunoprecipitated with a 120-kDa tyrosine-phosphorylated protein of unknown identity. In addition to supporting the notion that Src-related enzymes regulate the recruitment of Zap-70 in TCR signaling, these data added further complexity to previous models of regulation of Zap-70. Furthermore, they suggested that p120 may be an effector and/or a regulator of Zap-70 in activated T-lymphocytes.

Unique catalytic properties dictate the enhanced function of p59fynT, the hemopoietic cell-specific isoform of the Fyn tyrosine protein kinase, in T cells

As a result of alternative splicing, the fyn gene encodes two different tyrosine protein kinase isoforms. While one protein (p59fynB) is abundantly expressed in the brain, the alternative product (p59fynT) is contained only in cells of hemopoietic lineages, especially T lymphocytes. Sequence analyses have revealed that these two isoforms differ exclusively within a stretch of 52 amino acids which overlaps the end of the Src homology 2 (SH2) motif and the beginning of the catalytic domain. Consistent with the idea that FynT provides a specialized function in hemopoietic cells, we have previously shown that expression of activated FynT molecules, but not that of activated FynB polypeptides, enhanced the antigen responsiveness of a mouse T-cell line (BI-141) (D. Davidson, L. M. L. Chow, M. Fournel, and A. Veillette, J. Exp. Med. 175:1483-1492, 1992). In this study, we examined the basis for the distinct signalling capabilities of the two Fyn isoforms in T lymphocytes. Our biochemical analyses revealed that FynT is more adept than FynB at promoting antigen receptor-triggered calcium fluxes. This phenomenon likely contributes to the improved biological function of FynT during antigen stimulation, as the calcium ionophore ionomycin partially rescued the inability of FynB to enhance antigen-induced lymphokine secretion. To establish the structural basis for these observations, we also created and analyzed a series of chimeras of FynT and FynB. These studies demonstrated that the distinct catalytic domain of FynT, and not its altered SH2 motif, is responsible for the improved ability to augment antigen responsiveness. Similarly, this sequence enhances the ability to mobilize cytosolic calcium in response to antigen receptor stimulation. Taken together, these data show that the distinct biological impacts of FynT and FynB in T cells are related to limited structural differences in the amino-terminal portion of their catalytic domains and that they reflect, at least in part, the greater ability of FynT to mobilize cytoplasmic calcium.

Unique catalytic properties dictate the enhanced function of p59fynT, the hemopoietic cell-specific isoform of the Fyn tyrosine protein kinase, in T cells

As a result of alternative splicing, the fyn gene encodes two different tyrosine protein kinase isoforms. While one protein (p59fynB) is abundantly expressed in the brain, the alternative product (p59fynT) is contained only in cells of hemopoietic lineages, especially T lymphocytes. Sequence analyses have revealed that these two isoforms differ exclusively within a stretch of 52 amino acids which overlaps the end of the Src homology 2 (SH2) motif and the beginning of the catalytic domain. Consistent with the idea that FynT provides a specialized function in hemopoietic cells, we have previously shown that expression of activated FynT molecules, but not that of activated FynB polypeptides, enhanced the antigen responsiveness of a mouse T-cell line (BI-141) (D. Davidson, L. M. L. Chow, M. Fournel, and A. Veillette, J. Exp. Med. 175:1483-1492, 1992). In this study, we examined the basis for the distinct signalling capabilities of the two Fyn isoforms in T lymphocytes. Our biochemical analyses revealed that FynT is more adept than FynB at promoting antigen receptor-triggered calcium fluxes. This phenomenon likely contributes to the improved biological function of FynT during antigen stimulation, as the calcium ionophore ionomycin partially rescued the inability of FynB to enhance antigen-induced lymphokine secretion. To establish the structural basis for these observations, we also created and analyzed a series of chimeras of FynT and FynB. These studies demonstrated that the distinct catalytic domain of FynT, and not its altered SH2 motif, is responsible for the improved ability to augment antigen responsiveness. Similarly, this sequence enhances the ability to mobilize cytosolic calcium in response to antigen receptor stimulation. Taken together, these data show that the distinct biological impacts of FynT and FynB in T cells are related to limited structural differences in the amino-terminal portion of their catalytic domains and that they reflect, at least in part, the greater ability of FynT to mobilize cytoplasmic calcium.

CD4-mediated enhancement or inhibition of T cell activation does not require the CD4:p56lck association

CD4 is the coreceptor molecule expressed on the surface of T cells specific for or restricted by class II molecules of the major histocompatibility complex (MHC). Its expression on T cells is required for an optimal response to antigen (Ag). Three mechanisms have been invoked for the involvement of CD4 in T cell activation. First, it was shown that CD4 binds to MHC class II molecules on antigen presenting cells (APCs) thereby favoring an adhesion between effector cells and APCs. Association of CD4 to the T cell receptor and to the tyrosine kinase p56lck have also been shown to be critically involved in the positive function of CD4. Here, we demonstrate that the interaction of CD4 with p56lck is not required to enhance the response of two CD4-dependent, Ag-specific T cell hybridomas. Mutant forms of CD4 (TCD4), which lose association to p56lck, were expressed in these T cells and were shown to enhance the Ag-specific response as efficiently as the wild-type CD4. Moreover both CD4-dependent and independent T cell responses were inhibited by CD4-specific mAbs even when CD4 was not associated with p56lck. These results indicate that mechanisms distinct from sequestration of p56lck and/or negative signaling operate in these inhibitions. Results demonstrating enhancement of TCR-mediated signaling by the coaggregation of TCD4 mutant to the TCR further confirm that the association of p56lck to CD4 is not absolutely required for the regulatory functions of CD4. Our results suggest that the mechanisms implicated in the enhancement of T cell stimulation via CD4 depend solely on the extracellular and transmembrane domains of CD4.

The SH2 domain is required for stable phosphorylation of p56lck at tyrosine 505, the negative regulatory site

The catalytic function of Src-related tyrosine protein kinases is repressed by phosphorylation of a conserved carboxy-terminal tyrosine residue. Recent studies suggest that this inhibitory event is not the result of autophosphorylation but that it is mediated by another cytoplasmic tyrosine protein kinase, termed p50csk. In this report, we have evaluated the processes regulating the extent of phosphorylation of the inhibitory carboxy-terminal tyrosine residue of p56lck, a lymphocyte-specific member of the Src family. By analyzing kinase-defective variants of p56lck expressed in mouse NIH 3T3 cells, we have found that the noncatalytic Src homology 2 (SH2) domain, but not the SH3 sequence or the sites of Lck myristylation and autophosphorylation, is necessary for stable phosphorylation at the carboxy-terminal tyrosine 505. Further studies in which Lck and Csk were coexpressed in S. cerevisiae indicated that the absence of the SH2 domain did not affect the ability of Csk to phosphorylate p56lck at tyrosine 505. However, we observed that incubation of cells with the tyrosine phosphatase inhibitor pervanadate restored the tyrosine 505 phosphorylation of Lck polypeptides devoid of the SH2 motif. Additionally, the presence of the SH2 sequence protected tyrosine 505 from in vitro dephosphorylation by the hemopoietic tyrosine protein phosphatase CD45. Taken together, these findings raised the possibility that the SH2 motif contributes to the physiological suppression of the catalytic function of p56lck at least in part through its ability to stabilize phosphorylation at the inhibitory site.

The SH2 domain is required for stable phosphorylation of p56lck at tyrosine 505, the negative regulatory site

The catalytic function of Src-related tyrosine protein kinases is repressed by phosphorylation of a conserved carboxy-terminal tyrosine residue. Recent studies suggest that this inhibitory event is not the result of autophosphorylation but that it is mediated by another cytoplasmic tyrosine protein kinase, termed p50csk. In this report, we have evaluated the processes regulating the extent of phosphorylation of the inhibitory carboxy-terminal tyrosine residue of p56lck, a lymphocyte-specific member of the Src family. By analyzing kinase-defective variants of p56lck expressed in mouse NIH 3T3 cells, we have found that the noncatalytic Src homology 2 (SH2) domain, but not the SH3 sequence or the sites of Lck myristylation and autophosphorylation, is necessary for stable phosphorylation at the carboxy-terminal tyrosine 505. Further studies in which Lck and Csk were coexpressed in S. cerevisiae indicated that the absence of the SH2 domain did not affect the ability of Csk to phosphorylate p56lck at tyrosine 505. However, we observed that incubation of cells with the tyrosine phosphatase inhibitor pervanadate restored the tyrosine 505 phosphorylation of Lck polypeptides devoid of the SH2 motif. Additionally, the presence of the SH2 sequence protected tyrosine 505 from in vitro dephosphorylation by the hemopoietic tyrosine protein phosphatase CD45. Taken together, these findings raised the possibility that the SH2 motif contributes to the physiological suppression of the catalytic function of p56lck at least in part through its ability to stabilize phosphorylation at the inhibitory site.

Negative regulation of T-cell receptor signalling by tyrosine protein kinase p50csk

Tyrosine protein phosphorylation is necessary for antigen receptor-mediated activation of T lymphocytes. This signal is generated at least in part by the Src-related tyrosine protein kinases p56lck and p59fynT (refs 2, 3). The activity of these two enzymes is repressed by phosphorylation of a conserved carboxy-terminal tyrosine residue. Recent studies suggest that this inhibitory phosphorylation may be caused by p50csk (for C-terminal Src kinase), a tyrosine protein kinase which accumulates most abundantly in thymus and spleen. To investigate the function of Csk in T lymphocytes and characterize the processes regulating T-cell receptor (TCR) signalling, we examined the effects of overexpression of Csk on the physiology of an antigen-specific mouse T-cell line. We report here that p50csk negatively regulates TCR-induced tyrosine protein phosphorylation and lymphokine production. This provides evidence for the involvement of Csk in the regulation of T-cell activation.

The efficient bovine insulin presentation capacity of bone marrow-derived macrophages activated by granulocyte-macrophage colony-stimulating factor correlates with a high level of intracellular reducing thiols

Bone marrow-derived macrophages (BMM phi) were shown before to function as antigen-presenting cells. We show here, that the antigen presentation capacity of BMM phi depends on the nature of the antigen and is differently regulated by the lymphokines interferon-gamma (IFN-gamma) and granulocyte/macrophage-colony-stimulating factor (GM-CSF). When bovine insulin (BI) was employed as antigen, only BMM phi treated with GM-CSF (GM-CSF-M phi) were efficient presenters, but when presentation of the antigens ovalbumin and conalbumin was tested, IFN-gamma-pulsed BMM phi (IFN-gamma-M phi) proved superior to GM-CSF-M phi. The lack of efficient BI presentation function of IFN-gamma-M phi was only obvious, when native BI was used as antigen. Preprocessed BI was presented by IFN-gamma-M phi with drastically higher efficiency than by GM-CSF-M phi. Because processing of insulin depends on reduction of disulfide bonds, we analyzed the content of intracellular reducing thiols within IFN-gamma-M phi, GM-CSF-M phi, and untreated BMM phi. Only after stimulation with GM-CSF did the amount of reduced glutathione and cysteine strongly increase, while IFN-gamma did not efficiently augment the intracellular content of both thiols. These findings suggest that the lymphokines IFN-gamma and GM-CSF differently interfere with the processing capacity of BMM phi by differently regulating the intracellular concentration of the thiols reduced glutathione and cysteine. A high level of these thiols induced by GM-CSF correlates with a prominent capacity to present the antigen bovine insulin.

Structural requirements for enhancement of T-cell responsiveness by the lymphocyte-specific tyrosine protein kinase p56lck

To understand the mechanism(s) by which p56lck participates in T-cell receptor (TCR) signalling, we have examined the effects of mutations in known regulatory domains of p56lck on the ability of F505 p56lck to enhance the responsiveness of an antigen-specific murine T-cell hybridoma. A mutation of the amino-terminal site of myristylation (glycine 2), which prevents stable association of p56lck with the plasma membrane, completely abolished the ability of F505 p56lck to enhance TCR-induced tyrosine protein phosphorylation. Alteration of the major site of in vitro autophosphorylation, tyrosine 394, to phenylalanine diminished the enhancement of TCR-induced tyrosine protein phosphorylation by F505 p56lck. Such a finding is consistent with the previous demonstration that this site is required for full activation of p56lck by mutation of tyrosine 505. Strikingly, deletion of the noncatalytic Src homology domain 2, but not of the Src homology domain 3, markedly reduced the improvement of TCR-induced tyrosine protein phosphorylation by F505 Lck. Additional studies revealed that all the mutations tested, including deletion of the Src homology 3 region, abrogated the enhancement of antigen-triggered interleukin-2 production by F505 p56lck, thus implying more stringent requirements for augmentation of antigen responsiveness by F505 Lck. Finally, it was also observed that expression of F505 p56lck greatly increased TCR-induced tyrosine phosphorylation of phospholipase C-gamma 1, raising the possibility that phospholipase C-gamma 1 may be a substrate for p56lck in T lymphocytes. Our results indicate that p56lck regulates T-cell antigen receptor signalling through a complex process requiring multiple distinct structural domains of the protein.

Structural requirements for enhancement of T-cell responsiveness by the lymphocyte-specific tyrosine protein kinase p56lck

To understand the mechanism(s) by which p56lck participates in T-cell receptor (TCR) signalling, we have examined the effects of mutations in known regulatory domains of p56lck on the ability of F505 p56lck to enhance the responsiveness of an antigen-specific murine T-cell hybridoma. A mutation of the amino-terminal site of myristylation (glycine 2), which prevents stable association of p56lck with the plasma membrane, completely abolished the ability of F505 p56lck to enhance TCR-induced tyrosine protein phosphorylation. Alteration of the major site of in vitro autophosphorylation, tyrosine 394, to phenylalanine diminished the enhancement of TCR-induced tyrosine protein phosphorylation by F505 p56lck. Such a finding is consistent with the previous demonstration that this site is required for full activation of p56lck by mutation of tyrosine 505. Strikingly, deletion of the noncatalytic Src homology domain 2, but not of the Src homology domain 3, markedly reduced the improvement of TCR-induced tyrosine protein phosphorylation by F505 Lck. Additional studies revealed that all the mutations tested, including deletion of the Src homology 3 region, abrogated the enhancement of antigen-triggered interleukin-2 production by F505 p56lck, thus implying more stringent requirements for augmentation of antigen responsiveness by F505 Lck. Finally, it was also observed that expression of F505 p56lck greatly increased TCR-induced tyrosine phosphorylation of phospholipase C-gamma 1, raising the possibility that phospholipase C-gamma 1 may be a substrate for p56lck in T lymphocytes. Our results indicate that p56lck regulates T-cell antigen receptor signalling through a complex process requiring multiple distinct structural domains of the protein.

Differential regulation of T cell antigen responsiveness by isoforms of the src-related tyrosine protein kinase p59fyn

Recent observations suggest that the src-related tyrosine protein kinase p59fyn may be involved in antigen-induced T lymphocyte activation. As a result of alternative splicing, p59fyn exists as two isoforms that differ exclusively within a short sequence spanning the end of the Src Homology 2 (SH2) region and the beginning of the tyrosine protein kinase domain. While one p59fyn isoform (fynB) is highly expressed in brain, the alternative product (fynT) is principally found in T lymphocytes. To further understand the role of p59fyn in T cell activation and to test the hypothesis that p59fynT serves a tissue-specific function in T lymphocytes, we have examined the effects of expression of activated versions (tyrosine 528 to phenylalanine 528 mutants) of either form of p59fyn on the physiology of an antigen-specific mouse T cell hybridoma. Our results demonstrated that the two forms of fyn, expressed in equivalent amounts, efficiently enhanced antibody-induced T cell receptor (TCR)-mediated signals. In contrast, only p59fynT increased interleukin 2 production in response to antigen stimulation. This finding implies that the distinct p59fyn isoform expressed in T lymphocytes regulates the coupling of TCR stimulation by antigen/major histocompatibility complex to lymphokine production.

Presentation of insulin and insulin A chain peptides to mouse T cells: involvement of cysteine residues

The requirements for insulin presentation and recognition by A alpha b A beta b- and A alpha b A beta k-restricted mouse T cells were studied using a variety of derivatives of the insulin A chain. It was found that A chain peptides with irreversibly blocked Cys residues are non-stimulatory for the T cells. This suggests that at least one of the Cys residues is essential for recognition. On the other hand, all A chain peptides containing Cys residues modified in a way reversible by reaction with thiols are stimulatory yet differ in antigenic potency. All these A chain derivatives including a 14 amino acid fragment require uptake by antigen presenting cells (APC) for efficient presentation. Differences in stimulatory potency between the A chain peptides derived from the same insulin appear to be mainly due to the efficiency of uptake and/or processing by APC. Based on these findings we propose that processing in the case of insulin and its A chain derivatives involves the reductive deblocking of Cys residues or the rearrangement of disulfide bonds apart from a possible proteolytic cleavage. The same may apply to other proteins if Cys residues in the presented peptides are important for the interaction with Ia or the T cell receptor.

Adhesion versus coreceptor function of CD4 and CD8: role of the cytoplasmic tail in coreceptor activity

CD4 and CD8 play an important role in T-cell recognition and activation; however, their mechanisms of action are not well understood. We compare the effects of expressing CD4 and CD8 alpha either individually or together in a class II-restricted T-cell hybridoma. We also compare the effects of expressing truncated forms of CD4 or CD8 alpha that do not have a cytoplasmic tail and thus do not associate with the T-cell-specific tyrosine kinase p56lck, which has been implicated in T-cell activation. We demonstrate that, although CD4 and CD8 alpha can specifically enhance interleukin 2 secretion, maximal potentiation occurs with expression of CD4, which, unlike CD8, can bind to the same major histocompatibility complex protein as the T-cell receptor. Our data further indicate that the cytoplasmic tail and/or the associated p56lck are primarily significant for interleukin 2 secretion by the hybridomas we have examined when CD4 or CD8 can bind to the same major histocompatibility complex ligand as the T-cell receptor.

Enhancement of T-cell responsiveness by the lymphocyte-specific tyrosine protein kinase p56lck

Lymphocyte-specific tyrosine protein kinase p56lck is physically associated with CD4 and CD8 T-cell surface molecules, suggesting that it may transduce CD4/CD8-triggered tyrosine phosphorylation signals during antigen stimulation. Indeed, antibody-mediated aggregation of CD4 (to mimic interaction with its ligand, major histocompatibility complex (MHC) class II molecules), rapidly elevates the kinase activity of p56lck and is associated with marked changes in tyrosine protein phosphorylation. Genetic analyses suggest that the interaction of CD4/CD8 with p56lck results in a positive signal during antigen-induced T-cell activation. To evaluate directly the role of p56lck in T-cell activation, we introduced a constitutively activated form of Lck protein (tyrosine 505 to phenylalanine 505 mutant); in a CD4-negative, MHC-class II restricted mouse T-cell hybridoma. We report here that, as for transfection of CD4, expression of the Lck mutant enhanced T-lymphocyte responsiveness. This finding provides direct evidence that p56lck can positively regulate T-cell functions and that it mediates at least some of the effects of CD4 and CD8 on T-cell activation.

Equivalence of human and mouse CD4 in enhancing antigen responses by a mouse class II-restricted T cell hybridoma

We have examined the ability of hCD4 to interact functionally with mouse class II MHC molecules using the mouse T cell hybridoma BI-141, specific for beef insulin. We have previously shown that expression of mouse CD4 results in a marked enhancement of IL-2 release by BI-141 cells in response to beef insulin or, in a cross-reactive response, to pork insulin, on the appropriate mouse APCs. We now demonstrate that expression of hCD4 results in an equivalent stimulation of antigen responses by this mouse T cell hybridoma. The specificity of this effect was demonstrated by mAb and gp120 blocking studies. These data provide the first direct evidence for function of hCD4 and in an exclusively mouse system.

Insulin B chain functions as an effective competitor of antigen presentation via peptide homologies present in the thymus

The B chain of mammalian insulins contains appropriately spaced amino acids that predict recognition by T cells. However, all T cell clones from an HLA-DR1, Dw6 diabetic donor recognize epitopes associated with the A chain, and the B chain was found to inhibit these responses. Effective intramolecular competition at the level of the APC, not a direct effect on the T cell, is responsible for the inhibition. Insulin B chain contains two clusters of amino acid homology with the TCR beta chain and B chain peptides lacking these clusters do not compete for antigen presentation. A hole in the repertoire for T cells that recognize this portion of the insulin molecule may arise in the thymus by deletion of T cells that recognize similar peptides.

Processing requirements for the recognition of insulin fragments by murine T cells

In this study we investigated aspects of antigen processing using insulin and insulin A chain-derived fragments as model antigens in Ab alpha Ak beta-restricted T-cell stimulation. Similarly to other proteins, the immunodominant region of insulin recognized by these T cells is limited in size. It is located on the insulin A chain and encompasses a portion of the molecule that is represented faithfully by peptide A1-14(SSO3-)3. Efficient presentation of intact insulin and its entire A chain is dependent on uptake and processing by APC. Whereas peptides stemming from various globular proteins are known to be presented to T cells by APC without requiring processing, this is not the case with A-chain fragment A1-14 (SSO3-)3. This observation suggested that, in addition to proteolytic degradation, other mechanisms might play a role in the processing of these antigens. Three cys-residues are located in close proximity to those amino acid residues of the insulin A chain that are inferred to participate in the specific interaction with MHC class II molecules and the TcR. In A-chain derivatives that are stimulatory for the T cells or in intact insulin these cys residues are engaged in disulfide bonds or are S-sulfonated. Both linkages can be reversibly modified by reaction with thiols. Functional data indicate that from intact insulin and from structurally distinct A-chain derivatives a closely similar or identical peptide is formed and bound to class II molecules for recognition by the T cells. The question arises as to whether, in this processed peptide, the cys residues are present in reduced form, engaged in disulfide bonds, or are modified in some other way. Taken together, these findings suggest that modification of cys residues or isomerization of disulfide bonds may play a role in insulin processing. It can be expected that other proteins carrying cys residues in their immunodominant peptides may show similar processing requirements. The inhibition of N-glycosylation of proteins by tunicamycin in APC blocked the processing and presentation of insulin and OvA whereas, under the same conditions, the presentation of a processing-independent peptide was not affected. Furthermore, an autoreactive T-cell clone was capable of recognizing tunicamycin-treated APC. Since the expression of class II molecules was found to be unaltered as demonstrated by cytofluorometric analysis the deficient N-glycosylation appears to have little influence on class II antigen-mediated T-cell recognition but interferes with uptake of antigen and/or its processing by APC.

The specificity of the interaction between the agretope of an antigen and an Ia-molecule can depend on the T cell clonotype

A series of T cell clones was developed from (B10 x B10.BR)F1 mice immunized with the isolated A chain of pig insulin. The T cell clones show considerable diversity as defined by their distinct reactivities to pig, beef, sheep and horse insulins in combination with the same syngeneic Ab alpha Ak beta molecules. These species variants of insulin differ from each other only in amino acid residues in position A8, A9 or A10 within the so-called A chain loop and responsiveness of mice to these variants is under Ir gene control. A detailed analysis of the stimulatory capacity of various insulin/Ia combinations including inhibition experiments with anti-Ia- and -L3T4 antibodies led to the following interpretation: the amino acid residues A8-A10 are involved in the interaction of the insulin A chain with the Ia molecules. This region can, therefore, be regarded as part of the agretope. Structural variations within this region can modify the stimulatory potency of the insulin variants. However, whether a particular amino acid substitution results in an enhancement or a reduction of the response depends on the fine specificity of the T cell clone involved. Thus, an interaction of Ia molecules with antigen cannot solely account for the functional specificity of an agretope, rather this also depends on the structure of the particular T cell receptor that participates in recognition.

Acquisition of an additional antigen specificity after mouse CD4 gene transfer into a T helper hybridoma

We have transfected the mouse CD4 gene into a beef insulin (BI)-specific murine T helper hybridoma that lacks CD4 surface expression. The CD4-expressing transfectants have acquired an additional reactivity for pork insulin (PI), which was not detectable in the original recipient cell. The transfectants' response to PI can be completely abrogated by anti-CD4 antibodies. The transfected clone showed a 50-fold increased sensitivity towards BI in comparison to the same CD4- hybridoma. These experiments suggest that CD4 may be important in determining the antigen fine specificity and, therefore, may also play a role in altering the T cell repertoire.

Identification of transcripts of the T cell antigen receptor beta chain gene and major histocompatibility complex class II genes in antigen-presenting cloned BK-BI-2.6.C6 cells

The cloned murine cell line BK-BI-2.6.C6 has previously been shown to exhibit T cell characteristics, to synthesize and express MHC class II molecules, and to present protein antigens to antigen-dependent T cell clones. As a more definitive proof of the T-cell nature of these cells, transcripts of the rearranged T cell antigen receptor (TcR) beta gene were assessed by Northern blot analysis. BK-BI-2.6.C6 cells constitutively transcribe mRNA for the light chain of TcR and express the disulphide-linked alpha, beta TcR heterodimer at the cell surface. In addition mRNA for the polymorphic MHC class II subunits A alpha and A beta as well as for the invariant gamma chain were detected. BK-BI-2.6.C6 T cells effectively stimulated bovine insulin-reactive T hybridoma cells to lymphokine production in the presence of this antigen. Since the antigen-presenting and the responding T cell populations are maintained in culture in the absence of feeder cells, contamination by conventional accessory cells is excluded. These data unequivocally demonstrate that cloned murine Ia-expressing T cells can act as antigen-presenting accessory cells.

Synthesis and expression of MHC class II molecules in the absence of attached invariant chains by recombinant-interferon-gamma-activated bone-marrow-derived macrophages

Pure populations of in vitro propagated bone marrow-derived macrophages are constitutively Ia negative. Co-culturing of these cells with recombinant interferon-gamma (rIFN-gamma) resulted in the appearance of high amounts of Ia antigens at the cell surface of essentially all cells. The continuous presence of the stimulus was a prerequisite for sustained Ia expression because removal of the stimulus resulted in rapid decline of surface Ia. Two-dimensional (2D) gel analysis (1D isoelectric focusing, 2D sodium dodecyl sulfate-polyacrylamide gel electrophoresis) of class II molecules synthesized by rIFN-gamma-stimulated bone marrow macrophages (BMM phi) revealed that, in contrast to class II complexes hitherto described, BMM phi-derived I-A and I-E subregion-encoded subunits are synthesized without invariant chains. The invariant chain-deficient alpha,beta heterodimers are expressed at the cell surface in high proportions demonstrating that their correct assembly and transport to the cell surface is accomplished in the absence of invariant chains. The lack of invariant chains appears not to be due to a failure of rIFN-gamma to induce transcription of the gamma-chain gene because rIFN-gamma-induced, in contrast to uninduced, BMM phi accumulate high levels of invariant chain-specific transcripts as evidenced by Northern blot analysis. These findings suggest that translation of gamma-chain-specific mRNA is blocked in BMM phi for as yet unknown reasons. Alternatively, newly synthesized gamma chains might have escaped their regular intracellular maturation pathway as a result of unidentified modifications mediated by altered post-translational processing mechanisms.

Functional requirements of (Phe, G)-A--L-specific T-cell clones of (H-2b X H-2k)F1 origin

T-cell clones specific for the synthetic polypeptide antigen poly(LPhe, LGlu)-poly(DLAla)--poly(LLys) of (C57BL/6 X C3H/HeJ)F1 origin were tested for their biological activities. One group of clones was restricted in its proliferative response to the H-2b haplotype, the second to the H-2k haplotype, and the third to the F1 unique Ia determinants. All the clones which proliferated in response to antigen secreted interleukin-2 (IL-2) following stimulation. The H-2 restriction of the IL-2 secretion was the same as that of the proliferation. Two of the clones tested, C.6 and C.10, could provide help to B cells in antibody production. However, the genetic restriction profile of the helper activity was less stringent than that for the proliferative response. Thus, C.6, which proliferated in the presence of F1 antigen-presenting cells only, could help B cells and accessory cells of C3H/HeJ. C.10, which was restricted in its proliferative response to the H-2b haplotype, could collaborate with B cells and accessory cells of the H-2k haplotype as well. The antibody response of both clones was restricted to the parental or F1 strains.

Requirement of H-2 heterozygosity for autoimmunity in (NZB X NZW)F1 hybrid mice

In the F1 hybrid of autoimmune New Zealand Black (NZB) and phenotypically normal New Zealand White (NZW) mice, there occurs a severe systemic lupus erythematosus (SLE)-like autoimmune disease more fulminant than that found in the parental NZB mice. To determine the role of the H-2 complex in the pathogenesis of autoimmune disease of the (NZB X NZW)F1 hybrid, we developed H-2-congenic NZB (NZB.H-2z) and NZW (NZW.H-2d) strains, and compared the degree of autoimmune features between congenic H-2d/H-2d and H-2z/H-2z homozygous F1 hybrids and the original H-2d/H-2z heterozygous (NZB X NZW)F1 hybrid. We found that autoimmune features such as productions of IgG class anti-DNA antibodies and retroviral gp70 immune complexes and the development of renal disease were to a great extent reduced in both H-2 homozygous F1 hybrids, as compared with the H-2 heterozygous (NZB X NZW)F1 hybrid. It would thus appear that the heterozygosity of H-2d haplotype derived from NZB and H-2z from NZW is essential for the autoimmune disease characteristic of the (NZB X NZW)F1 hybrid.