Antigen recognition by helper T cells elicits a sequence of distinct changes of their shape and intracellular calcium

Helper T-cell activation is initiated in vivo when the T-cell receptor complex recognizes an antigen fragment associated with MHC class II molecules on the surface of an antigen-presenting cell. In most previous studies of this phenomenon, T cells were stimulated not with antigen-presenting cells, but with CD3-specific antibodies. This approach provided considerable understanding of the cascade of molecular events triggered by T-cell receptor stimulation. However, the specific consequences of cell-cell interactions are still poorly understood. We therefore used a dual imaging system that provides simultaneous transmission and fluorescence images to study the morphological changes and variations of intracellular calcium concentration ([Ca2+]i) triggered in a human CD4+ antigen-specific T-cell clone in response to antigen presented by a class II-transfected murine fibroblast.

T cells loaded with the Ca(2+)-sensitive fluorescent dye Fura-2 were individually monitored for half an hour following their contact with a monolayer of antigen-pulsed antigen-presenting cells. The response was found to have three distinct phases. During the first few minutes after contact, the T cell moves over the antigen-presenting cells, as if 'scanning' them. After several minutes, an oscillating [Ca2+]i response begins, accompanied by the immobilization of the cell and the retraction of pseudopodia. This rounding-up was probably Ca(2+)-dependent, as it could also be triggered by ionomycin or thapsigargin. Later during the [Ca2+]i response, the T cell becomes flattened and further elongated, suggesting increased adhesion to antigen-presenting cells.

The physiological signal for T-cell activation, antigen recognition, is a three-step process reminiscent of the three steps previously observed in the interaction between neutrophils and endothelial cells. During these successive steps, a mobile, weakly interacting T cell is transformed into an immobile cell fully engaged in the activation pathway. Thus, antigenic recognition is not instantaneous, but evolves slowly by progressive amplification of the signal given by a few antigen molecules, eventually resulting in T-cell activation.