José María Rojo

CD4+ T Cells: Specificity and Function

The majority of T lymphocytes, and those best-characterized at the present time, recognize foreign antigens as peptide fragments associated with self major histocompalibility complex (MHC)-encoded proteins. The reason for this preoccupation of T cells with MHC-encoded cell surface molecules is not presently understood, and controversy surrounds many aspects of MHC restriction in T cell specificity. Both T cells and MHC molecules can be subdivided into two classes. MHC class I molecules are found on the surface of most somatic cells, while MHC class II molecules are expressed selectively on the surfaces of cells involved in immune responses, such as B cells and macrophages. T cells can be subdivided by the cell surface expression of the CD4 and CD8 markers: CD4 T cells recognize foreign protein antigen fragments associated with self class II MHC molecules, while CDS T cells recognize foreign protein antigen fragments associated with self class I MHC molecules. T cells also respond to polymorphic differences in MHC molecules. Recognition of allogeneic or non-self MHC molecules largely follows the pattern of foreign antigen recognition, with CD4 T cells predominantly recognizing class II MHC polymorphisms, and CD8 T cells largely recognizing class I MHC polymorphisms. These responses to non-self MHC molecules involve a very high proportion of normal T cells, it being estimated that 1-10% of T cells will respond to a particular non-self MHC molecule. This surprising commitment of T cells to recognition of MHC molecules not normally encountered requires a biologically satisfying explanation. These fmdings raise several questions: First, why is expression of CD4 associated with recognition of foreign antigen in association with self class II MHC molecules? Second, how can one account for the high frequency of alloreactive T cells? And third, why

The co-receptor function of murine CD4.

Peripheral T cells can be subdivided into two major sets based on the differential expression of the cell surface molecules CD4 and CD8. It was noted early that the CD4 T-cell subset recognized antigen presented by class II MHC molecules, while the CD8 subset recognized antigen presented by class 1 MHC molecules (Swain 1983). We will term this phenomenon the specificity association; the association of CD4 expression with class II MHC restricted antigen recognition is fare more absolute than is the association with helper function (Janeway et al. 1988). The observed specificity association has led to two suggestions: First, that CD4 and CD8 bind to non-polymorphic portions of class II and class I MHC molecules, respectively, and second, that CD4 and CD8 are accessory molecules whose major function is to augment the interaction of T cells with antigenpresenting cells bearing tbe appropriate MHC molecule (Dialynas et al. 1983). In this review, data will be presented that supports the alternative view that the major function of CD4 is in forming a part of the T-cell receptor for its complex class II MHC ligand (Saizawa et al. 1987a, Janeway et al. 1988, Janeway 1989). By inference, CD8 would serve a similar role in class I MHC recognition. Because our data point of CD4 as a physical component of the T-cell receptor (TCR) for antigenic peptides bound to class II MHC molecules, we believe that the term co-receptor (Janeway 1988) more accurately reflects the role of this molecule in T-cell activation and specificity than does the term accessory molecule,

A simple method for the radioactive iodination of CD4 molecules

A method for the vectorial radioiodination of CD4 and other membrane proteins having few or no tyrosine residues in the extracytoplasmic domains is described. Incubation of the cells with sulfosuccinimidyl (hydroxyphenyl) propionate (sulfo-SHPP), a water-soluble derivative of the Bolton-Hunter reagent results in the coupling of hydroxyphenyl groups to free amino groups of cell surface proteins and these groups are then vectorially radioiodinated using 1,3,4,6-tetrachloro-3 alpha,6 alpha-diphenylglycoluril (Iodogen). The method is highly efficient, gentle, fast, simple, and does not require the previous radiolabelling of the Bolton-Hunter reagent.

Complement regulatory protein Crry/p65 costimulation expands natural Treg cells with enhanced suppressive properties in proteoglycan-induced arthritis

OBJECTIVEnTo investigate the costimulatory role of Crry/p65 (Crry), a membrane complement regulatory protein, on the expansion and function of natural Treg cells and their ability to ameliorate proteoglycan-induced arthritis (PGIA), an animal model of inflammatory arthritis in which the role of natural Treg cells is not well established.nnnMETHODSnCD4+CD25+ natural Treg cells from BALB/c mice were activated in vitro and costimulated by Crry. The expanded cells were phenotypically characterized, and their suppressive effect on T cell proliferation was assayed in vitro. The potential prophylactic and therapeutic effects of this population versus those of natural Treg cells in PGIA were studied. The clinical score, histology, the antigen-specific isotype antibody pattern, in vitro T cell responses, and the presence of Treg cells in the paws were studied.nnnRESULTSnCrry costimulation enhanced the in vitro expansion of natural Treg cells while maintaining their phenotypic and suppressive properties. Crry-expanded Treg cells had stronger suppressive properties in vivo and a longer ameliorating effect in the PGIA model than did natural Treg cells. Crry-expanded Treg cells suppressed T cell- and B cell-dependent responses in PGIA, changing the pathogenic antibody isotype pattern and decreasing antigen-dependent secretion of cytokines, including interferon-γ, interleukin-12 (IL-12), and IL-17. Increased FoxP3 expression was detected in the paws of mice transferred with Crry-expanded Treg cells.nnnCONCLUSIONnCrry-mediated costimulation facilitates in vitro expansion of natural Treg cells while maintaining their suppressive properties in vitro and in vivo in the PGIA model. These results highlight the potential of the complement regulatory protein Crry to costimulate and expand natural Treg cells capable of suppressing disease in an animal model of chronic inflammatory arthritis.

Loss of N-terminal Charged Residues of Mouse CD3ϵ Chains Generates Isoforms Modulating Antigen T Cell Receptor-mediated Signals and T Cell Receptor-CD3 Interactions

The antigen T cell receptor (TCR)-CD3 complexes present on the cell surface of CD4+ T lymphocytes and T cell lines express CD3ϵ chain isoforms with different isoelectric points (pI), with important structural and functional consequences. The pI values of the isoforms fit the predicted pI values of CD3ϵ chains lacking one, two, and three negatively charged amino acid residues present in the N-terminal region. Different T cells have different ratios of CD3ϵ chain isoforms. At a high pI, degraded CD3ϵ isoforms can be better recognized by certain anti-CD3 monoclonal antibodies such as YCD3-1, the ability of which to bind to the TCR-CD3 complex is directly correlated with the pI of CD3ϵ. The abundance of CD3ϵ isoforms can be modified by treatment of T cells with the proteinase inhibitor phenanthroline. In addition, these CD3ϵ isoforms have functional importance. This is shown, first, by the different structure of TCR-CD3 complexes in cells possessing different amounts of isoforms (as observed in surface biotinylation experiments), by their different antigen responses, and by the stronger interaction between low pI CD3ϵ isoforms and the TCR. Second, incubation of cells with phenanthroline diminished the proportion of degraded high pI CD3ϵ isoforms, but also the ability of the cells to deliver early TCR activation signals. Third, cells expressing mutant CD3ϵ chains lacking N-terminal acid residues showed facilitated recognition by antibody YCD3-1 and enhanced TCR-mediated activation. Furthermore, the binding avidity of antibody YCD3-1 was different in distinct thymus populations. These results suggest that changes in CD3ϵ N-terminal chains might help to fine-tune the response of the TCR to its ligands in distinct activation situations or in thymus selection.

The TCR/CD3 Complex: Opening the Gate to Successful Vaccination

The success of vaccination is directly or indirectly based on the specificity of antigen recognition by T lymphocytes, their efficient activation and expansion, and the generation of vaccine-specific effectors and memory cells. These traits are largely dependent on the correct assembly and expression of sufficient number of functional TCR/CD3 complexes in the cell surface. In this review, some of the genetic and epigenetic factors that determine the correct assembly and structure of the TCR/CD3 complex are summarized. Those physiologic or pathologic factors leading to natural variations, or pathologic alterations of the standard that might lead to poor response to vaccination and that could give some possibilities to pharmacological intervention are emphasized.

Recognition of MHC Class II Antigens by the CD4: T Cell Receptor Complex

T lymphocytes that express CD4 respond to foreign antigens presented in the context of syngeneic class II MHC molecules. T cells responding to allogeneic class II MHC molecules generally express CD4, but exceptions are observed. We show that the same receptor on a cloned T cell line recognizes both foreign antigen associated with self-class II MHC and nonself-class II MHC molecules. The role of CD4 in these recognition responses appears to be to bind to class II MHC molecules and, through association with the T cell receptor recognizing the same class II MHC molecule, to effectively activate the T cell. It is proposed on the basis of these two findings that the high ligand multiplicity of alloantigens recognized in unmodified form by high affinity anti-nonself-MHC receptors can explain the finding of CD8+ T cells responding to nonself-class II MHC molecules, as well as the inability of anti—CD4 to inhibit such nonself-class II MHC recognition completely. Finally, data will be presented to suggest that class II may also play a role in selecting the functional activity elicited in response to a protein antigen in vivo. These data show that the CD4:T cell receptor complex interaction with the complex of foreign antigen and self-MHC is the critical event in initiating most immune responses and may also play a role in controlling their functional outcome.