Frequency encoding of T-cell receptor engagement dynamics in calcium time series

The sustained increase of the cytosolic calcium concentration ([Ca2+](i)) plays a central role in T-cell receptor (TCR)-mediated T-cell activation. Previous experiments using a [Ca2+](i) clamp technique have demonstrated that specificity is encoded by the [Ca2+](i) oscillation frequency since cytokine transcription factors are activated in a frequency-dependent manner. An outstanding question is how encoding of specific activation occurs under physiological conditions. In this case, continuous TCR interactions with specific peptides bound to cell surface-associated major histocompatibility complexes are driving the sustained [Ca2+](i) increase. Addressing this question, we analyzed [Ca2+](i) time series from individual T-cells mathematically. We are able to identify signal fluctuations associated with the TCR-triggering dynamics. We also find that [Ca2+](i) time series associated with T-cells activated to IFN-gamma production exhibit oscillations with higher frequencies than the time series corresponding to T-cells not activated to IFN-gamma production. We show that signal autocorrelations are a means to distinguish functional signals according to their associated cytokine production. The signal level, however, allows for the distinction of nonfunctional from functional signals. These findings provide strong evidence for specificity encoding of biological functions in intracellular signals via signal level and signal correlations.