Mature CD4؉ and CD8 ؉ T lymphocytes are believed to build and express essentially identical surface ␣␤ T-cell receptor-CD3 (TCR⅐CD3) complexes. However, TCR⅐CD3 expression has been shown to be more impaired in CD8 ؉ cells than in CD4؉ cells when CD3␥ is absent in humans or mice. We have addressed this paradox by performing a detailed phenotypical and biochemical analysis of the TCR⅐CD3 complex in human CD3␥-deficient CD8؉ and CD4 ؉ T cells. The results indicated that the membrane TCR⅐CD3 complex of CD8 ؉ T lymphocytes was conformationally different from that of CD4 ؉ lymphocytes in the absence of CD3␥. In addition, CD8؉ , but not CD4 ؉ , CD3␥-deficient T lymphocytes were shown to contain abnormally glycosylated TCR␤ proteins, together with a smaller, abnormal TCR chain (probably incompletely processed TCR␣). These results suggest the existence of hitherto unrecognized biochemical differences between mature CD4؉ and CD8 ؉ T lymphocytes in the intracellular control of ␣␤TCR⅐CD3 assembly, maturation, or transport that are revealed when CD3␥ is absent. Such lineage-specific differences may be important in receptor-coreceptor interactions during antigen recognition.Mature ␣␤ T lymphocytes recognize pathogen-derived peptides on antigen-presenting cells by means of the multimeric membrane protein ensemble termed the T-cell receptor (TCR) 1 ⅐CD3 complex. This TCR⅐CD3 complex includes two clonally distributed variable chains that directly interact with antigens (TCR␣ and TCR␤) and four invariant polypeptides that regulate assembly and signal transduction (CD3␥, CD3␦, CD3⑀, and ) (1). The assembly of complete TCR⅐CD3⅐ complexes takes place in a highly ordered manner within the endoplasmic reticulum: first CD3 chains, then TCR chains, and finally chains. Further conformational maturation, including carbohydrate processing, occurs in the Golgi apparatus before exportation of mature complexes to the T cell surface. The biochemical machinery involved in the assembly, processing, and exportation of TCR⅐CD3 complexes is assumed to be shared by all ␣␤ T-lineage cells. Thus, CD4ϩ and CD8 ϩ are believed to build biochemically and conformationally identical antigen receptors, although differences in the numbers that reach or remain at the cell surface have been noted (2). Therefore, the lack of any CD3 chain would be expected to affect to a similar extent the assembly and exportation of TCR⅐CD3 complexes by mature CD4 ϩ and CD8 ϩ T cells. However, this was not the case in several murine and human CD3 deficiencies (reviewed in Ref.3). In particular, it has been consistently shown that in the absence of CD3␥ or CD3␦, TCR⅐CD3 expression (or conformation) is more impaired in mature peripheral CD8 ϩ cells than in their CD4 ϩ counterparts, both in human and in murine deficiencies (3-7). Three other observations suggested the existence of CD8 ϩ cell-specific defects in human CD3␥ deficiency: first, the proband died after a viral infection (a cytolytic T-celldependent function) despite normal antibody responses (helper T-cell-dependent) (8...

SUMMARY Common variable immunodeficiency (CVID) is a very frequent but heterogeneous syndrome of antibody formation. The primary defect remains unknown, but many reports describe peripheral blood T lymphocyte dysfunctions in a substantial proportion of CVID patients, which may impair T–B cell collaboration. In order to investigate whether such putative defects were intrinsic to T cells or, rather, secondary to quantitative differences in T cell subset distribution, or to other described disorders, we have used Herpesvirus saimiri (HVS) for the targeted transformation of CVID CD4+ and CD8+ T cells and subsequent functional evaluation by flow cytometry of their capacity to generate cell surface (CD154, CD69) or soluble (IL‐2, TNF‐α, IFN‐γ) help after CD3 engagement. Unexpectedly, the results showed that 40 different CVID blood samples exposed to HVS gave rise with a significantly increased frequency to transformed CD4+ T cell lines, compared to 40 age‐matched controls (27%versus 3%, P≤ 0·00002) suggesting the existence of a CVID‐specific signalling difference which affects CD4+ cell transformation efficiency. The functional analysis of 10 CD4+ and 15 CD8+ pure transformed T cell lines from CVID patients did not reveal any statistically significant difference as compared to controls. However, half of the CD4+ transformed cell lines showed CD154 (but not CD69) induction (mean value of 46·8%) under the lower limit of the normal controls (mean value of 82·4%, P≤ 0·0001). Exactly the same five cell lines showed, in addition, a significantly low induction of IL‐2 (P≤ 0·04), but not of TNF‐α or IFN‐γ. None of these differences were observed in the remaining CD4+ cell lines or in any of the transformed CD8+ cell lines. We conclude that certain CVID patients show selective and intrinsic impairments for the generation of cell surface and soluble help by CD4+ T cells, which may be relevant for B lymphocyte function. The transformed T cell lines will be useful to establish the biochemical mechanisms responsible for the described impairments.

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CD3 proteins may have redundant as well as specific contributions to the intracellular propagation and final effector responses of TCR-mediated signals at different checkpoints during T cell differentiation. We report here on the participation of CD3 gamma in the activation and effector function of human mature T lymphocytes at the antigen recognition checkpoint. Following TCR-CD3 engagement of human CD3 gamma-deficient T cell lines, and despite their lower TCR-CD3 surface levels compared to normal controls, mature T cell responses such as protein tyrosine phosphorylation and the regulation of expression of several cell surface molecules, including the TCR-CD3 itself, were either normal or only slightly affected. In contrast, other physiological responses like the specific adhesion and concomitant cell polarization on ICAM-1-coated dishes were selectively defective, and activation-induced cell death was increased. Our data indicate that CD3 gamma contributes essential specialized signaling functions to certain mature T cell responses. Failure to generate appropriate interactions may abort cytoskeleton reorganization and initiate an apoptotic response.

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