TcR:CD3 Transduced Calcium Signalling In Response To T Cell Antigen Presentation.
A. Tárnok*, R. Headgrave2), P. Robinson1), R. Lechler2), G. Lombardi2), Department of Lymphocyte Development, 1)Department of Transplantation Biology, Clinical Sciences Center, and 2)Rheumatology Unit, Hammersmith Hospital, RPMS, London W12 ONN, UK.
Activated human T cells express HLA classII molecules and thus acquire the capacity to present antigens to other T cells. However, T cell antigen presentation induces nonresponsiveness (anergy) in IL-2 secreting T cells. We have investigated the mechanism for this phenomenon. Flow-cytometric analysis of the intracellular events in recipient T cells (RTC) following recognition of antigen presented by T cells (PTC) revealed that there is a transient Ca2+ mobilization in 30-40% of the RTC. The transient lasted for 1h, the mean [Ca2+]i increased by 150 nM over basal level. PTC induced [Ca2+]i increase was completely abolished by removal of extracellular Ca2+ with EGTA and reduced after blocking of Ca2+-channels with Ni2+ or Co2+. The Ca2+ elevation was essential for induction of anergy. Stimulation in the absence of extracellular Ca2+ did not anergize RTC.
Calcium changes were further investigated by confocal microscopy. Fluo3 stained RTC were stimulated with FuraRed stained PTC. The majority of RTC increased their [Ca2+]i after contact with PTC. Analysis of single cells showed a heterogeneity of [Ca2+]i kinetics: single and multiple responses and [Ca2+]i oscillations were found. This heterogeneity in the kinetics could explain the low percentage of reponders found by flow cytometry. In addition, PTC that induced a [Ca2+]i increase also transiently increased their [Ca2+]i.
Taken together these findings suggest that T cell presentation induces an altered TcR:CD3-transduced signal. Given that PTC express high levels of B7, LFA-3, and ICAM-1 and induce Ca2+ flux in RTC, the failure of PTC as stimulatory cells is clearly not due to a lack of conventional costimulation. The possible mechanisms underlying this altered signalling are currently under investigation.