Antigen receptors on T and B lymphocytes: parallels in organization and function

Immunohistochemical Evaluation of Mononuclear Infiltrates in Canine Lupoid Onychodystrophy

Claw biopsy samples of 11 dogs with lupoid onychodystrophy were evaluated. They were stained with hematoxylin and eosin and with antibodies against CD 3 as a T-cell marker, BLA 36 and HM 57 (CD 79α) as B-cell markers, and lysozyme, Mac 387, and major histocompatibility complex (MHC) class II as a marker for histiocytes using an immunoperoxidase and avidin-biotin technique. Inflammatory cells were counted in five high-power fields. The inflammatory infiltrate comprised predominantly B cells and T cells. Macrophages were typically only present in small numbers. CD 3, BLA 36, lysozyme, and MHC class II preserved significant antigenicity during formalin fixation and short decalcification for 24–48 hours, whereas CD 79α and particularly Mac 387 seemed to be more susceptible to denaturation by the decalcification process.

Immunomodulatory Effects of Dietary Exposure to Selected Polycyclic Aromatic Hydrocarbons in the Bluegill (Lepomis macrochirus)

Polycyclic aromatic hydrocarbons (PAH) have been demonstrated to affect immune system modulation. The freshwater species of fish, Lepomis macrochirus (bluegill), was employed to investigate the effects of a 14-day dietary exposure to PAH including 2-aminoanthracene (2-AA), 2-methylnaphthalene (2-MN), and 9,10-dimethylanthracene (9,10-DMA) and a mixture of these 3 compounds at a total dose of 3.1 ± 0.01 mg on lymphocyte proliferation stimulated with 3 mitogens (concanavalin A [Con A], phorbol ester, and calcium ionophore). 2-Aminoanthracene was mitogenic itself and with added mitogens. 2-Methylnaphthalene induced some stimulatory and some inhibitory effects upon cell proliferation by Con A. 9,10-DMA and the mixture each suppressed cell proliferation. The mixture was highly suppressive to lymphocytes. Intracellular baseline calcium levels were reduced, possibly as a step prior to cell death. All PAH compounds tested were immunomodulatory to bluegill lymphocytes. Bluegill were demonstrated to have utility as a biomarker species for investigation of immunotoxicity.

Analysis of the T-cell receptor repertoire of synovial T-cells

The recognition of a wide diversity of antigens by lymphocytes is made possible by the expression of a large range of highly variable antigen specific receptors, coded for by tandem arrays of genes, which undergo rearrangement during T- and B-cell development. The study of T-cell receptor (TCR) diversity and clonal composition of mixed T-cell populations has taken advantage of the features of the TCR molecule in various ways. This chapter focuses on the study of T-cells obtained from the synovial fluid of patients with inflammatory arthritis. Methods to process and store the samples and to separate cell populations are described. Two alternative molecular methods to analyse TCR diversity, identify clonal expansions, and track specific T-cell populations over both time and location are also detailed.

Lymphoma immunophenotyping: a new era in paraffin-section immunohistochemistry

Recent advances in immunohistochemistry have made it possible to investigate lymphomas for the expression of a wide range of antigens in fixed tissues. Epitope retrieval, sensitive detection methods, and the availability of new monoclonal antibodies have all contributed to one's ability to perform detailed immunophenotyping that previously could only be done in cryostat sections or by flow cytometry. Current lymphoma classifications make use of characteristic immunophenotypic profiles that aid in the reproducible diagnosis and subcassification of these neoplasms. The following is a review of the current state of immunophenotyping for lymphoid neoplasms in fixed tissues.

Biased V beta usage in immature thymocytes is independent of DJ beta proximity and pT alpha pairing

During thymus development, the TCR beta locus rearranges before the TCR alpha locus. Pairing of productively rearranged TCR beta-chains with an invariant pT alpha chain leads to the formation of a pre-TCR and subsequent expansion of immature pre-T cells. Essentially nothing is known about the TCR V beta repertoire in pre-T cells before or after the expression of a pre-TCR. Using intracellular staining, we show here that the TCR V beta repertoire is significantly biased at the earliest developmental stage in which VDJ beta rearrangement has occurred. Moreover (and in contrast to the V(H) repertoire in immature B cells), V beta repertoire biases in immature T cells do not reflect proximity of V beta gene segments to the DJ beta cluster, nor do they depend upon preferential V beta pairing with the pT alpha chain. We conclude that V gene repertoires in developing T and B cells are controlled by partially distinct mechanisms.

1H-NMR analysis of CD3-epsilon reveals the presence of turn-helix structures around the ITAM motif in an otherwise random coil cytoplasmic tail

The conformation adopted in solution by the cytoplasmic tail of CD3-epsilon has been analyzed by 1H-nmr. The cytoplasmic tail is mostly random coil expect for the amino acids conforming the immunoreceptor tyrosine-based activation motif (ITAM), YxxL/IxxxxxxxY xxL. Although the N-terminal Y xxL sequence of the motif is poorly folded, adopting 6-residue turn-like conformations with the Tyr side chain in two different orientations, the C-terminal Y xxL sequence is placed in a more complex structure involving a set of nonclassical alpha-helix turns and beta-turns that comprises 11 amino acids. This structure is not modified by phosphorylation of the tyrosine residue. The differences in the conformation adopted around the two tyrosines of the ITAM motif suggest that they may play different roles pertaining to either binding signal transducing proteins or, alternatively, proteins involved in other processes such as endoplasmic reticulum location.

Flow cytometric detection of CD79a expression in T-cell acute lymphoblastic leukemias

We evaluated the lineage specificity of CD79a in acute leukemias using 3-color flow cytometry in 58 consecutive cases. A panel of cell-surface antigens, including myeloid-associated markers, B-cell-associated markers, and T-cell-associated markers, was used. All cases of acute myeloid leukemia were CD79a-, whereas all cases of B-lineage acute lymphoblastic leukemia (ALL) were CD79a+. Three of 8 cases of T-cell ALL showed variable CD79a expression, indicating the presence of a blast subset expressing a relatively high level of CD79a. We investigated the clinical and pathologic characteristics of these 3 cases. All 3 cases had L1 or L2 morphology and expressed surface CD3. None of the other B-cell-associated markers were positive, although 1 case expressed CD13 and CD33. Uncommon random karyotypic abnormalities were identified in all 3 cases. Molecular studies demonstrated monoclonal gene rearrangement of T-cell receptor gamma in 2 of 3 cases. All 3 patients were 18 years old or younger; 1 patient did not enter remission, and 1 had disease relapse in 8 months. Our findings provide further support for the existence of a subset of T-cell ALL coexpressing CD3 and CD79a. Further study of the clinical and biologic significance of this subset may be warranted.

von Willebrand factor is the most reliable immunohistochemical marker for megakaryocytes of myelodysplastic syndrome and chronic myeloproliferative disorders

To find the best immunohistochemical marker for megakaryocytes in normal marrow, myelodysplastic syndrome (MDS), and chronic myeloproliferative disorders (CMPD), 57 marrow biopsy specimens were studied semiquantitatively with immunohistochemical methods using a panel of 7 antibodies. The staining intensity was graded 0 to 3 for scoring 100 consecutive megakaryocytes in each stained section. The final score for each stain was the sum of these 100 megakaryocytes individually multiplied by their corresponding grade. In normal marrow (11 cases), the average scores for antivon Willebrand factor (vWF) and Ulex europaeus agglutinin-1 (UEA-1) were 177.1 and 195.1, respectively. The scores for the other 5 markers, including anti-platelet-derived growth factor-BB, 2 anti-transforming growth factor-beta 3, anti-CD61, and anti-CD79a ranged from 96.1 to 124.1. In MDS (27 cases), the scores were 200.8 (vWF), 152.6 (UEA-1), and 28.7 to 98.5 (others). In CMPD (19 cases), the scores were 220.5 (vWF), 179.2 (UAE-1), and 64.8 to 101.2 (others). These results show that vWF and UEA-1 are good immunohistochemical markers for megakaryocytes in normal marrow, and vWF is the best marker in MDS and CMPD. For routine practice, vWF is the most reliable marker for identifying atypical megakaryocytes, especially in the cases of 5q-syndrome and agnogenic myeloid metaplasia.

Gene rearrangements in T-cell lymphoblastic lymphoma

This study presents an examination of the Ig heavy chain (IgH) and T-cell receptor gamma (TCRgamma) genes in a series of 39 CD3-positive T-cell acute lymphoblastic leukaemia (ALL) cases with and without co-expression of CD79a; 30/39 cases had a rearrangement of the TCRgamma genes and two of these 30 cases also demonstrated an IgH rearrangement. No cases had solely an IgH rearrangement. The conclusion of the study is that lymphoblastic lymphoma cases that are positive for CD3 are of T-cell lineage, regardless of CD79a expression.

T cell signaling: a decision of life and death

T lymphocytes constitute an essential part of the immune system. Their generation, activation, proliferation but also survival is subject to tight regulation by several extracellular factors including cytokines, MHC-antigen complexes and co-stimulatory ligands. The balanced interplay between these factors determines the fate of the T cell. Both in thymic development and in a peripheral immune response, triggering of the T cell antigen receptor (TCR) through interaction with the MHC-antigen complex can result in T cell proliferation. However, in the absence of co-stimulatory signals from antigen-presenting cells a state of non-responsiveness is induced that is called anergy. In addition, stimulation of the TCR on activated T cells or thymocytes can lead to the induction of apoptosis. Here we will give an overview of the intracellular signal transduction pathways that are activated by the stimuli that dictate the fate of a T cell as they were presented at the International Symposium on soluble HLA antigens held in 1997 in Brussels.

Co-expression of CD79a (JCB117) and CD3 by lymphoblastic lymphoma

Acute lymphoblastic leukaemia/lymphoma is a malignant disorder derived from the clonal proliferation of lymphoid precursor cells. Whether the tumour cells are of B- or T-cell type is an important criterion for prognosis which has not been available previously to pathologists, due to the lack of a reliable early B-cell marker functioning on routinely processed material. This has changed with the production of monoclonal antibodies against the B-cell signalling molecule CD79a. CD79a is expressed on normal and neoplastic B cells from the early stages of B-cell maturation and has been considered to be B-cell-specific. Currently available antibodies against CD79a, in particular JCB117, allow the identification of B cells, and hence B lymphoblastic disease, in paraffin-embedded material. In this study, the expression of CD79a (JCB117) and CD3 has been investigated in 149 cases of T and 68 cases of B lymphoblastic leukaemia/lymphoma. For the first time, co-expression of CD79a (JCB117) and CD3 is reported in 10 per cent of cases of T lymphoblastic leukaemia/lymphoma. This finding raises questions about the co-expression of T- and B-cell markers in the development of lymphocytes, benign as well as malignant, and alerts pathologists to a potential problem in diagnosis.

Differential chronology of TCRADV2 gene use by alpha and delta chains of the mouse TCR

The genes coding for TCR alpha and delta chains share the same genetic locus (TCRA/D). The rules governing the utilization of a V gene with the alpha and delta chains have not been established. More specifically, it is not known whether the position of a gene within the locus influences its utilization in alpha and delta TCR. To elucidate these points, we mapped ADV2 genes in the TCRA/D locus of BALB/c mice and analyzed their utilization in TCR alpha and delta transcripts from thymi isolated from mice of different ages. Our results show that all ADV2 genes can be used by the two chains, but with strikingly different patterns. Moreover, ADV2 utilization by the alpha chain proceeds in successive concentric waves during development, suggesting a progressive regulation of gene accessibility and utilization. These results support independent control of TCRA and TCRD gene assembly.

A murine monoclonal antibody (928) recognizing a new epitope formed with a combination of HLA-DPA1*0201 and DPB1*0301 gene products

A murine monoclonal antibody (mAb), 928, that recognizes a cell surface antigen (928 Ag) on a human Epstein-Barr virus-transformed fetal liver-derived lymphoid progenitor cell line (FL4.4) was generated. The 928 mAb reacted with only FL4.4; it did not react with any other 57 cell lines tested. Two color flowcytometry analysis of peripheral blood mononuclear cells (PBMC) revealed that the 928 mAb reacted with B cell and monocyte fractions from only two individuals out of 63 unrelated donors. Biochemical analyses showed that the 928 Ag composes of two molecules (33 and 34 Kd) and forms a SDS-resistant, noncovalently linked dimer conformation, the feature being similar to that of peptide-bound MHC class II molecules. Treatment of FL4.4 cells with the 928 mAb significantly facilitated homotypic cell aggregation. In addition, treatment of PBMC of the 928 Ag+ donor with recombinant IL-4 augmented the expression of the 928 Ag on CD64+ monocytes. Typing of HLA-DRB1, DPA1 and DPB1 alleles of the 928 Ag expressing and nonexpressing cells revealed that the 928 Ag is expressed only on PBMC of HLA-DPA1*0201 and DPB1*0301 positive donors. Finally, anti-DP antibody precleared 928 Ag from the cell lysate. These results demonstrate that the 928 mAb recognizes a polymorphic determinant of HLA-DPA1*0201-DPB1*0301 gene products. The possibility that amino acids in the groove of the peptide-binding site of HLA-DP molecules are critical for the 928 epitope is discussed.

T cell receptor alpha gene rearrangement and transcription in adult thymic gamma delta cells

T cells belong to two separate lineages based on surface expression of alpha beta or gamma delta T cell receptors (TCR). Since during thymus development TCR beta, gamma, and delta genes rearrange before alpha genes, and gamma delta cells appear earlier than alpha beta cells, it has been assumed that gamma delta cells are devoid of TCR alpha rearrangements. We show here that this is not the case, since mature adult, but not fetal, thymic gamma delta cells undergo VJ alpha rearrangements more frequently than immature alpha beta lineage thymic precursors. Sequence analysis shows VJ alpha rearrangements in gamma delta cells to be mostly (70%) nonproductive. Furthermore, VJ alpha rearrangements in gamma delta cells are transcribed normally and, as shown by analysis of TCR beta-/- mice, occur independently of productive VDJ beta rearrangements. These data are interpreted in the context of a model in which precursors of alpha beta and gamma delta cells differ in their ability to express a functional pre-TCR complex.

Assembled pre-B cell receptor complexes are retained in the endoplasmic reticulum by a mechanism that is not selective for the pseudo-light chain

The pre-B cell receptor (BCR) complex, consisting of micro heavy chain, a pseudo-light chain, and the Mb-1/B29 heterodimer, directs the transition to the mature B cell stage. Plasma membrane expression of the pre-BCR is extremely low, despite its presumed signaling function. We have compared assembly and intracellular transport of the pre-BCR complex with that of the BCR complex in mature B cells. Synthesis and assembly rate of pre-BCR and BCR components are comparable. However, the pre-BCR is subject to a highly efficient retention mechanism, which only allows exit of a few percent of the complexes from the endoplasmic reticulum (ER). This small transported pool of pre-BCR complexes is significantly enriched for protein-tyrosine kinase activity, as compared with the ER-localized receptor pool. Accordingly, the Src-related tyrosine kinase Lyn was found in the transported glycoprotein fraction but not in association with ER-localized glycoproteins. Upon introduction of a conventional light chain into pre-B cells, plasma membrane receptor levels increased, but the efficiency of intracellular transport of the receptor complex was not restored to that in mature B cells. This indicates that the ER retention mechanism is not selective for the pseudo-light chain and may be inherent to pre-B cells. We propose that this retention mechanism contributes to the regulation of pre-BCR-mediated signal transduction.

A role for Bruton's tyrosine kinase in B cell antigen receptor-mediated activation of phospholipase C-gamma 2

Defects in the gene encoding Bruton's tyrosine kinase (Btk) result in a disease called X-linked agammaglobulinemia, in which there is a profound decrease of mature B cells due to a block in B cell development. Recent studies have shown that Btk is tyrosine phosphorylated and activated upon B cell antigen receptor (BCR) stimulation. To elucidate the functions of this kinase, we examined BCR signaling of DT40 B cells deficient in Btk. Tyrosine phosphorylation of phospholipase C (PLC)-gamma 2 upon receptor stimulation was significantly reduced in the mutant cells, leading to the loss of both BCR-coupled phosphatidylinositol hydrolysis and calcium mobilization. Pleckstrin homology and Src-homology 2 domains of Btk were required for PLC-gamma 2 activation. Since Syk is also required for the BCR-induced PLC-gamma 2 activation, our findings indicate that PLC-gamma 2 activation is regulated by Btk and Syk through their concerted actions.

Direct assessment of junctional diversity in rearranged T cell receptor beta chain encoding genes by combined heteroduplex and single strand conformation polymorphism (SSCP) analysis

In order to define the extent of T cell heterogeneity and clonality, unique DNA sequences in the junctional region in rearranged T cell receptor (TcR) genes can be studied. For this purpose we have adapted a non-denaturing nucleic acid gel electrophoresis procedure to detect TcR junctional diversity. Detection of junctional diversity is based upon electrophoretic separation of single stranded (ss) and double stranded (ds) DNA molecules via mobility shifts due to nucleotide sequence polymorphism. To examine the capacity of this nucleic acid gel electrophoresis procedure to detect nucleotide sequence polymorphism in the CDR 3 region within TcR V beta gene family sequences polymerase chain reaction (PCR) amplified TcR V beta 5.1/5.4 and V beta 14 cDNA sequences were analyzed. The results of this study showed that (1) the single strand conformation polymorphism (SSCP) procedure has a low capacity to discriminate between diverse TcR V beta cDNA sequences due to comigration of the ssDNA molecules, which results in an underestimation of the heterogeneity in a given T cell population; (2) comigrating ssDNA and/or dsDNA (homoduplex) molecules can be separated by the formation of heteroduplex molecules; these heteroduplex molecules provide essential additional information on the degree of nucleotide sequence polymorphism in the CDR 3 region within the TcR V beta cDNA sequences; (3) the double strand conformation polymorphism (DSCP) procedure provides a fast and reliable procedure to detect junctional diversity within the sequences tested. Using DSCP a more detailed assessment of amplified TcR V beta cDNA sequences can be obtained as compared with SSCP analysis only. Data obtained by gel analysis were very similar to those obtained by conventional bacterial cloning and DNA sequencing procedures on the corresponding cDNA clones. In conclusion, this new gel electrophoresis procedure allows a direct assessment of the extent of T cell heterogeneity and clonality by screening junctional diversity in TcR chain encoding sequences in clinical conditions with (oligo)clonal expansion of T lymphocytes.

Composition and function of T-cell receptor and B-cell receptor complexes on precursor lymphocytes

The precursor T-cell receptors (TCRs) and B-cell receptors (BCRs) direct lymphocyte development to the mature T-cell and B-cell stage, respectively. Recent genetic and biochemical experiments reveal the striking parallel in structure and function of these receptors. They consist of TCR beta and BCR mu chains paired with surrogate TCR alpha and BCR light chains. Both receptors employ a two-component signal transduction unit: CD3 gamma epsilon for the pre-TCR, and CD79ab for the pre-BCR. Plasma membrane levels of pre-TCR/BCR complexes are kept extremely low, most probably by a mechanism involving specific retention in the endoplasmic reticulum. This mechanism may control the signalling activity of pre-TCR/BCR and therewith the lymphocyte differentiation process.

T cell receptor delta gene rearrangement and T early alpha (TEA) expression in immature alpha beta lineage thymocytes: implications for alpha beta/gamma delta lineage commitment

Mature T cells comprise two mutually exclusive lineages expressing heterodimeric alpha beta or gamma delta antigen receptors. During development, beta, gamma, and delta genes rearrange before alpha, and mature gamma delta cells arise in the thymus prior to alpha beta cells. The mechanism underlying commitment of immature T cells to the alpha beta or gamma delta lineage is controversial. Since the delta locus is located within the alpha locus, rearrangement of alpha genes leads to deletion of delta. We have examined the rearrangement status of the delta locus immediately prior to alpha rearrangement. We find that many thymic precursors of alpha beta cells undergo VDJ delta rearrangements. Furthermore, the same cells frequently coexpress sterile T early alpha (TEA) transcripts originating 3' of C delta and 5' of the most upstream J alpha, thus implying that individual alpha beta lineage cells undergo sequential VDJ delta and VJ alpha rearrangements. Finally, VDJ delta rearrangements in immature alpha beta cells appear to be random, supporting models in which alpha beta lineage commitment is determined independently of the rearrangement status at the TCR delta locus.

Role of alterations in Ca(2+)-associated signaling pathways in the immunotoxicity of polycyclic aromatic hydrocarbons

Polycyclic aromatic hydrocarbons (PAHs) are an important class of environmental pollutants that are known to be carcinogenic and immunotoxic. The effects of PAHs on the immune system of various animals and models have been studied for at least 30 yr. Despite these efforts, the mechanism or mechanisms by which PAHs exert their effects on the immune system are still largely unknown. During recent years, the molecular events associated with lymphocyte activation and receptor-mediated signaling have become increasingly clear. Substantial progress has been made in understanding the molecular and cellular bases for toxicant-induced immune cell injury. Understanding mechanisms of drug or chemical effects on the immune system is an important area of research in the field of immunotoxicology, and indeed in all fields of toxicology. Mechanistic toxicology plays an important role in risk assessment and extrapolation of potential human health effects. In this review, we have summarized recent evidence that has examined the effects of PAHs on the immune system of animals and humans. In particular, we have focused on the effects of PAHs on cell signaling in lymphoid cells and have examined the hypothesis that PAHs alter lymphocyte activation via calcium-dependent mechanisms. Previously published reports are discussed, and new data obtained with murine B cells and cell lines are presented demonstrating the relationship between alterations in intracellular calcium and immune dysregulation. These data demonstrate a strong association between PAH-induced alterations in B- and T-lymphocyte activation and changes in calcium homeostasis.

The Staphylococcus aureus enterotoxin B superantigen induces specific T cell receptor down-regulation by increasing its internalization

Superantigens are able to stimulate T lymphocyte populations expressing T cell antigen receptors (TCR) belonging to particular V beta families. Moreover, the presence of these superantigens may induce long term unresponsiveness (anergy) of these sensitive cells. Some bacterial toxins are potent superantigens. We have analyzed in vitro the capacity of some Staphylococcus aureus enterotoxin superantigens to modulate T cell antigen receptor expression and the cellular mechanisms involved. Staphylococcus enterotoxin B (SEB) induced rapid down-regulation of surface T cell antigen receptors in V beta 3-expressing T lymphocytes, as assessed by flow cytometry. This phenomenon was a consequence of the direct interaction between the toxin and the TCR since it was observed in the absence of cells expressing major histocompatibility complex class II molecules. The cellular mechanism involved in SEB-induced down-regulation of TCR was further investigated. Immunofluorescence and confocal microscopy experiments showed that toxin B induced intracellular accumulation of TCR.CD3 in endocytic vesicles. Moreover, SEB induced an increase in T cell receptor endocytosis as measured using radiolabeled Fab fragments of an anti-CD3 monoclonal antibody. Taken together, our observations indicate that Staphylococcus enterotoxin B superantigen induced changes in the dynamics of surface T cell receptors, which resulted in the fast reduction of membrane receptor numbers.

Denaturing and non-denaturing gel electrophoresis as methods for the detection of junctional diversity in rearranged T cell receptor sequences

Two nucleic acid gel electrophoresis techniques were tested as a possible tool for analyzing junctional diversity in rearranged T cell receptor (TcR) sequences in order to define the extent of T cell heterogeneity. For this purpose denaturing gradient gel electrophoresis (DGGE) as well as non-denaturing gel electrophoresis (nDGE) techniques have been studied. Detection of junctional diversity is based on mobility shifts, caused by nucleotide sequence polymorphism, of polymerase chain reaction amplified rearranged TcR sequences. DGGE as well as nDGE procedures are suitable for the detection of junctional diversity in TcR V gene family sequences based on sequence dependent separation. Compared to DGGE, nDGE of DNA is a relatively simple and rapid procedure, with a high separation potential. nDGE permits separation of double stranded (homoduplexes) and/or single stranded DNA molecules of the majority of TcR chain encoding sequences. Formation and detection of unique heteroduplex molecules combined with single stranded DNA molecule analysis in nDGE permits the recognition of the remaining sequences, thus providing additional information on the degree of T cell heterogeneity. In conclusion, these nucleic acid gel electrophoresis techniques allow a direct assessment of the heterogeneity and clonality of T cell populations by the detection of junctional diversity in TcR chain encoding sequences. This analysis can be performed without the need of cell propagation and/or cellular cloning procedures, thereby eliminating the risk of introducing technical artefacts.

Antigen processing: the gateway to the immune response

The T-lymphocyte response to an antigen is governed by the source of that antigen and the way in which it is processed. Before recognition by T lymphocytes, proteins must be degraded to peptides by antigen-presenting cells. The peptides are then presented on major histocompatibility complex (MHC) molecules for recognition by the T cells. Antigens arising outside the cell (e.g., bacteria) are phagocytosed and processed by the exogenous pathway for presentation on MHC class II molecules (e.g., DR) to CD4+ cells. Antigens derived from the cytoplasm (e.g., viral proteins) are processed by the endogenous pathway for presentation by MHC class I molecules (e.g., HLA-A, -B, -C) to CD8+ cells. The response to a hapten or drug is a function of the antigen processing pathway and is determined by its chemical properties. Antigen processing also governs the T-cell response to pathogens, vaccines, and autoimmune conditions.

CD3 components at the surface of pro-T cells can mediate pre-T cell development in vivo

Developmentally arrested pro-T cells (CD4-8-, IL-2R+, HSA++) of RAG-1-deficient mice appear to express low levels of CD3 molecules in the absence of T cell receptor (TcR) chains at their surface, while developmentally arrested pre-T cells of TcR alpha-deficient mice express low levels of a disulfide-linked TcR beta chain in association with CD3 molecules. Cross-linking of the CD3 modules on pro-T cells of RAG-1-/- mice in vivo, with either of two different CD3 epsilon-specific monoclonal antibodies, induces differentiation of these pro-T cells into pre-T cells (CD4+8+, IL-2R-, HSA+), concomitant with a rapid expansion of the thymic T cell compartment, up to 175-fold within 12 days. The same effects can be produced by introduction of a mutant TcR beta transgene lacking most of the variable domain (delta V-TcR beta) into the RAG-1-/- background. These experiments suggest that cross-linking of the CD3 modules on pro-T cells mimics the signaling function expected of the pre-TcR complex, which is found at the surface of pre-T cells prior to functional TcR alpha gene rearrangement. The variable domain of the TcR beta chain is apparently not essential for inducing these aspects of T cell development.