Protein Kinase Cθ Focusing at the cSMAC Is a Consequence rather than Cause of TCR Signaling and Is Dependent on the MEK/ERK Pathway

Correlation between protein kinase Ctheta focusing within the central supramolecular activation cluster (cSMAC) of the immunological synapse and optimal TCR/costimulatory receptor ligation was interpreted to imply that PKCtheta focusing is required for productive signaling. However, this notion has been called into question and competing data suggest that the cSMAC contributes to receptor down-modulation. The observation that PKCtheta focusing at the cSMAC is promoted by CD28 coligation, and also that it occurs late after proximal tyrosine phosphorylation events have been initiated, has led us to investigate an alternative possibility that PKCtheta focusing might be a consequence rather than a cause of productive integrated signaling. Indeed, we found that inhibition of the downstream signaling molecules MEK and PI3K (but not of calcineurin, NF-kappaB, JNK, or p38 MAPK) significantly prevented the focusing of PKCtheta at the cSMAC. It recently has been suggested that the cSMAC may be associated with TCR degradation and signal termination. Using MEK inhibition as a tool, we observed that absence of detectable PKCtheta focusing had no significant effect on TCR down-modulation or duration of CD3zeta phosphorylation. Our results suggest that PKCtheta focusing at the cSMAC occurs as a consequence of productive integrated downstream signaling at least at the level of MEK. If PKCtheta focusing accurately reflects the cSMAC as a whole, then our data also argue against the cSMAC as being required for proximal TCR signal termination.