Correlation between extracellular cytokine gradients and intracellular calcium dynamics in neutrophils

Abstract

In the immune system, neutrophil movement toward the site of inflammation by concentration gradient is essential. In our previous studies, concentration gradient of cytokine on the neutrophil membrane is experimentally quantified. The results showed that the concentration gradient on the neutrophil membrane repeats positive and negative values while neutrophil moving. However, the key gap remains unclear: how external, membrane-scale cytokine gradient dynamics are coupled to intracellular signalling that drives protrusion and propulsion, and what the observed phase relationship implies for chemotaxis. In this study, the dynamics of intracellular calcium (Ca²⁺) in neutrophils were measured, and these findings were compared with previously measured cytokine concentration gradients to verify their correlation. The results show three main findings: (1) Neither IL-8 stimulation nor neutrophil viability produced a statistically significant change in the global intracellular Ca²⁺-indicator intensity or its decay rate. (2) The dominant frequencies of the membrane cytokine gradient and the intracellular Ca²⁺ gradient were similar, suggesting that external gradient dynamics and intracellular signaling are linked. (3) A consistent phase difference was observed between cytokine and Ca²⁺gradients, indicating a delayed intracellular response to changes in membrane cytokine gradients. These findings support a mechanistic role of calcium signaling in neutrophil propulsion and provide quantitative constraints for chemotaxis models.