John Tite

Quantitative variation in la antigen expression plays a central role in immune regulation

The analysis of la antigen function has focused primarily on allelic variants of Ia molecules. In this review Charles Janeway and his colleagues discuss evidence that quantitative rather than qualitative variation in Ia antigen expression had a major role in immunoregulation and immunologically mediated disease.

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,

Comparison of expression of a humanized monoclonal antibody in mouse NSO myeloma cells and Chinese Hamster Ovary cells

We report a detailed comparison of two commonly used stable, amplifiable mammalian expression systems (Chinese Hamster Ovary cells/dihydrofolate reductase and Mouse NSO myeloma/glutamine synthetase) used to express a humanized IgG1 monoclonal antibody. We compare copy number and steady state mRNA levels of both the selectable marker and heavy chain of the antibody throughout the selection and amplification process. In both cell lines, copy number and steady state levels of heavy chain and selectable marker increased during selection and were further increased during amplification. As expected, an increase in steady state mRNA levels of heavy chain correlated with an increase in expression of antibody whilst an increase in the steady state levels of mRNA of the selectable marker correlated with increased resistance to the selective agent. In NSO and CHO cells producing equivalent amounts of antibody, the copy number of the antibody genes and selectable marker was significantly higher in the CHO cells than in the NSO cells. However, the steady state mRNA levels of the heavy chain of the antibody were virtually identical. Rates of protein secretion in the two cell lines were also compared and found to be very similar. When the antibody purified from both systems was compared in a number of functional assays they behaved identically.

Generation, Propagation, and Variation in Cloned, Antigen-Specific, Ia-Restricted Cytolytic T-Cell Lines

Since the demonstration that B lymphocytes bear surface immunoglobulin (Ig), while helper T lymphocytes are surface Ig negative but bear the T-cell differentiation antigen Thy-1, attempts have been made to correlate the function of a T cell with the expression of differentiation antigens by that cell. In the mouse and in man, it has been found that the majority of helper T cells bear the T4 (or L3T4a) marker, while suppressor and cytolytic T cells bear the T8 or Lyt-2 surface molecule (Cantor and Boyse 1976; Jandinski et al. 1976; Rheinherz et al. 1979). However, many studies, especially those employing cloned T-cell populations, appear to violate these “rules.” Thus, cells with helper, suppressor, or cytolytic capabilities have been assigned to either of these populations by several investigators (Swain et al. 1981; Thomas et al. 1981). In this report, we will dis-cuss our current work on cytolytic T cells that are antigen-specific, la-restricted, and L3T4a positive (Tite and Janeway 1984a, b; Tite et al. 1985). Such cells also manifest at least some of the functions associated with helper T cells. Our studies indicate that the critical determinant of the function of a T cell is the antigen- nonspecific molecules it produces upon activation by antigen:MHC, not its cell surface antigen phenotype or its specificity for a particular self-MHC molecule. We will present evidence that demonstrates that such cells are found amongst normal T cells, and that previous attempts to demonstrate such cells probably failed for technical reasons having to do with assay conditions and target-cell susceptibility.

The Development of Genetically Defined Live Bacterial Vaccines

It has long been recognised that vaccination with live organisms can induce more effective protection against infectious diseases than the use of dead vaccines presented in conjunction with currently available adjuvants. Live vaccines offer particular advantages in their ability to induce cell-mediated immune responses which are often critical for the establishment of solid protection against certain infectious agents. The route of vaccine delivery is also of importance. Oral vaccines can induce immune responses at mucosal surfaces and such responses are often lacking in individuals who receive parenteral vaccines. The combination of live organisms with oral delivery thus offers potential advantages for creating practical and efficacious vaccines. Many problems were experienced with early live vaccines because of the frequent occurrence of reversion to virulence and batch to batch variation. These problems were difficult to deal with because the attenuating lesions present in avirulent vaccine strains derived from virulent pathogens were uncharacterised at the genetic level. Recent advances in our understanding of the genetics and the mechanisms employed by pathogens to establish infections in the host has allowed the construction of genetically defined attenuated derivatives of many pathogens.

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.

Control of B Cell Activation by Helper T Cells

The activation of B cells to secrete antibody by helper T cells (Th) was one of the first cell interactions to be characterized in the immune response.1 The process has been extensively characterized since its initial description twenty years ago. Two features of this cellular interaction have attracted particular attention, the requirement for physical linkage of the determinant recognized by the B cell to the determinant recognized by the helper T cell2 and the requirement for identity in the I region of the major histocompatibility complex (MHC).3 These two features suggested that intimate contact between the helper T cell and the B cell were required for efficient B cell activation. The stringency of these requirements, and the failure to activate ‘bystander’ B cells in in vivo systems, further suggested that a signal might be transmitted to the B cell directly via cell:cell contact, for instance by aggregation of the B cell’s Ia glycoproteins.