CD28‐mediated induction of proliferation in resting T cells in vitro and in vivo without engagement of the T cell receptor: Evidence for functionally distinct forms of CD28

Abstract

JJ316 and JJ319 are rat CD28-specific monoclonal antibodies (mAb) of the gamma1 kappa isotype with identical co-stimulatory potency. At a concentration 100-1000-fold higher than that required for co-stimulation, JJ316, but not JJ319 induces massive proliferation of all T cell subsets in vitro without T cell receptor (TCR) triggering. "Direct" stimulation by JJ316 is fully blocked by JJ319, indicating that it is not due to cross-reactivity of JJ316 with the TCR complex or other activating receptors. JJ316 binds much more slowly to primary T cells than JJ319, whereas both antibodies bind with similar kinetics to CD28-transfected L-929 cells, suggesting that JJ316 binding to T cells requires redistribution or a conformational change of CD28. In vivo, JJ316 but not JJ319 induces rapid and transient proliferation of most CD4 T cells and, indirectly, of B cells. These data show that TCR engagement is not an absolute prerequisite either in vitro or in vivo for the induction of T cell proliferation through CD28 and suggest that mAb JJ316 is able to stimulate resting T cells directly by recruiting CD28 molecules from an inactive to an active form.

Efficient expansion of regulatory T cells in vitro and in vivo with a CD28 superagonist

“…5-7). In agreement with our earlier findings that all CD4 T cells undergo DNA synthesis in response to CD28 stimulation [32], we observed that also in the CD25 -subset, a high degree of cycling is observed on day 1 after stimulation ( Fig. 5).…”

“…This also holds true for the rat [31], where in addition, we have previously described a class of "superagonistic" CD28-specific mAb which are able to induce proliferation of all T cell subsets without the need for TCR engagement [32]. Importantly, CD28 superagonists are also active in vivo, where in response to a single mAb injection, CD4 T cell numbers in peripheral lymphoid organs rise fivefold within 3 days before returning to base line levels [32].…”

“…This also holds true for the rat [31], where in addition, we have previously described a class of "superagonistic" CD28-specific mAb which are able to induce proliferation of all T cell subsets without the need for TCR engagement [32]. Importantly, CD28 superagonists are also active in vivo, where in response to a single mAb injection, CD4 T cell numbers in peripheral lymphoid organs rise fivefold within 3 days before returning to base line levels [32]. Surprisingly, this dramatic polyclonal T cell response occurs without apparent inflammation or discomfort, a finding we have previously attributed to the strong induction of IL-10 but not of pro-inflammatory mediators during in vivo activation [33].…”

“…In addition to "conventional" TCR-and CD28-specific mAb which effectively costimulate rat T cell responses [31], we also tested whether the putative Treg cells defined by their CD4 + CD25 + CD45RC low CTLA-4 + phenotype can be activated by CD28 superagonists which induce T cell proliferation without TCR engagement [32].…”

Rôle de CTLA-4 dans la cosignalisation négative du système immunitaire

“…Une forme soluble de CTLA-4 (sCTLA-4) a été décrite, dont l'expression est augmentée dans les celle d'interleukine (IL)-2, l'expression du récepteur de l'IL-2 [3][4][5], la survie cellulaire via l'expression accrue des molécules anti-apoptotiques dont Bcl-X L [6]. Il existe des signaux activés par CD28 indé-pendamment de l'activation antigénique comme cela a été démontré par l'utilisation d'anticorps monoclonaux (Acm) anti-CD28 nommés superagonistes [7]. L'analyse des mécanismes transcriptionnels et post-transcriptionnels régulés par CD28 a permis de démontrer que cette molécule contrôle l'expression et l'activité des facteurs de transcription NFAT (nuclear factor of activated T-cells), NF-kB et AP-1 sur le promoteur du gène de l'IL-2 [8,9].…”