Developmental regulation of Lck targeting to the CD8 coreceptor controls signaling in naive and memory T cells.

The question of whether enhanced memory T cell responses are simply due to an increased frequency of specific cells or also to an improved response at the single cell level is widely debated. In this study, we analyzed T cell receptor (TCR) transgenic memory T cells and bona fide memory T cells isolated from virally infected normal mice using the tetramer technology. We found that memory T cells are qualitatively different from naive T cells due to a developmentally regulated rearrangement of the topology of the signaling machinery. In naive cytotoxic T cells, only a few CD8 molecules are associated with Lck and the kinase is homogeneously distributed inside the cell. However, in vivo priming of naive T cells induces the targeting of Lck to the CD8 coreceptor in the cell membrane and the consequent organization of a more efficient TCR signaling machinery in effector and memory cells.

Generation of chimeric monoclonal antibodies from mice that carry human immunoglobulin Cgamma1 heavy of Ckappa light chain gene segments.

Gene targeting in mouse embryonic stem (ES) cells was used to replace (i) the mouse immunoglobulin heavy chain (IgH) Cgamma2a gene segment (mCgamma2a) with the human Cgamma1 gene segment (hCgamma1), and (ii) the mouse immunoglobulin light chain (IgL) Ckappa gene segment (mC kappa) with its human counterpart (hC kappa). ES cells carrying these gene conversions were used to generate chimeric mice that transmitted the human alleles through the germ line. Mice homozygous for both gene alterations were generated by breeding. Serum from homozygous mutant mice contained comparable amounts of antibodies with chimeric kappa or mouse lambda light chains but only small fractions of basal serum IgG or antibodies elicited against immunizing agents contained chimeric heavy chains. A relative increase in immunogen-specific hCgamma1 antibodies was seen following immunization in combination with the saponin adjuvant QS-21. The effect of this was to shift the IgG1-dominated response to an IgG subclass profile that included significant amounts of IgG2a, IgG2b and IgG3 and chimeric IgG. The amounts of antibody secreted by hybridomas derived from mutant and wild-type mice were similar. Sequencing confirmed correct splicing of hCgamma1 and hCkappa gene segments to mouse J gene segments in hybridoma Ig gene transcripts. In conclusion, IgHhCgamma1/IgLhCkappa double mutant mice provide a useful animal model for deriving humanized antibodies with potential applications in immunotherapy and diagnostics in vivo as well as for investigating hCgamma1 associated functions.

A T cell receptor (TCR) antagonist competitively inhibits serial TCR triggering by low-affinity ligands, but does not affect triggering by high-affinity anti-CD3 antibodies.

It has been demonstrated that modified peptides which fail to induce detectable T cell responses can act as T cell receptor (TCR) antagonists when presented together with agonist by the same antigen-presenting cell (APC). We report that a TCR antagonist competitively inhibits TCR triggering induced by low-affinity ligands such as agonistic peptides or bacterial superantigens. However, the same antagonist cannot inhibit TCR triggering and T cell activation induced by high-affinity anti-CD3 antibodies that engage most TCR at once. These results indicate that TCR antagonists inhibit T cell responses by interfering with the ongoing process of serial triggering, rather than by delivering an inhibitory signal to T cells.

Quantitative contribution of CD4 and CD8 to T cell antigen receptor serial triggering.

CD4 and CD8 are thought to function as coreceptors by binding to the cognate major histocompatibility complex (MHC) molecules recognized by the T cell antigen receptor (TCR) and initiating the signal transduction cascade. We report that during T cell-antigen-presenting cell interaction, triggered TCRs and coreceptors are downregulated and degraded with identical kinetics. This coordinated disappearance takes place whenever the TCR is triggered, even when the coreceptor does not engage the cognate MHC molecule and is the consequence of binding of the coreceptor-associated Lck to ZAP-70. The interaction of coreceptor and cognate MHC molecules is dispensable when T cells are stimulated by optimal ligands, but becomes crucial when suboptimal ligands are used. In the latter case the coreceptor increases the efficiency of TCR triggering without changing the activation threshold or the quality of the T cell response.