Ca2+ mobilization in physiologically stimulated single T cells gradually increases with peptide concentration (analog signaling)

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Abstract
We have investigated Ca2+ mobilization in single T cells stimulated with their physiological ligand, i.e. antigenic peptide bound to major histocompatibility complex (MHC) molecules on antigen‐presenting cells (APC). Fibroblasts expressing I‐Ed class II molecules were pulsed with a peptide derived from the λ2315 immunoglobulin light chain. Onto such antigen‐pulsed fibroblasts were sedimented cloned Th1 cells loaded with Fura‐2. Changes in cytosolic Ca2+ concentration in single T cells were continually monitored by use of an imaging system based on fluorometry. Ca2+ mobilization was both peptide‐specific and MHC‐restricted. Within seconds of the initial APC‐T cell contact, a Ca2+ spike could be observed. The Ca2+ response gradually declined over a 25‐min period, during which oscillations were noted. Various parameters characterizing the magnitude of the Ca2+ response (latency, increase rate, max and mean Ca2+ increase, frequency and period of oscillations) all correlated with the amount of peptide used for pulsing the fibroblasts. Thus, Ca2+ mobilization in single T cells appears not to be an all or none phenomenon. Rather, activation is incremental (analog signaling), the degree of Ca2+ mobilization probably being related to the number of stimulatory peptide‐MHC complexes on the surface of the APC. The extent of calcium mobilization and lymphokine production (interleukin (IL)‐2, IL‐3, interferon‐γ) correlated, at least at the population level.

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