T cell receptor (TCR) engagement by an antigen presenting cell (APC) results in reorganization of intracellular and membrane molecules at the T/APC interface, forming a “signalosome”, the immunological synapse (IS). An early event associated with T/APC interaction is Ca2+ influx. K+ channels, Kv1.3 and KCa3.1, modulate Ca2+ signaling in human T cells. Resting and activated human T cells express both channels, albeit to different degrees. Kv1.3 channels modulate Ca2+ in resting, while KCa3.1 channels do so in activated, T cells. Although these channels play such an important role in Ca2+ homeostasis, very little is known about their localization in the IS. Furthermore, aberrant T cell responses in IS formation and Ca2+ influx have been documented in T cells from patients with systemic lupus erythematosus (SLE). The potential involvement of K+ channels in the etiology and progression of SLE remains unknown.
Herein we determined K+ channel membrane distribution in resting and activated human T cells following TCR engagement and performed comparative studies with SLE T cells to decipher the role of K+ channels in the Ca2+ response anomaly. Our data show that in SLE T cells, Kv1.3 channels constitute the dominant channel and are functionally identical to their normal counterparts. We also found that resting SLE T cells show faster Kv1.3 kinetics out of the IS as compared to healthy T cells and comparable to healthy pre-activated T cells. However, normal pre-activated T cells recruit and maintain KCa3.1 channels in the IS after Kv1.3 channels leave, while SLE T cells do not express the appropriate KCa3.1 channel number to support this activated phenotype. This Kv1.3 mobility defect appears to be specific to SLE and not other autoimmune diseases, as it was not observed in rheumatoid arthritis (RA) patients.
Further, transcription factor activation and gene expression relies on the shape of the Ca2+ response. Although SLE T cells demonstrate abnormal transcription factor regulation and gene expression, the potential contribution of the Ca2+ responses to these abnormalities remains unknown. We performed single T cell Ca2+ response analysis to better define the Ca2+ shape in SLE T cells. Our data suggest that there is an increase in cells with more sustained Ca2+ response in SLE as compared to normal and RA T cells, while a transient, short duration, Ca2+ response is more pronounced in normal T cells. We also found that during and upon termination of the Ca2+ response, Kv1.3 channels are retained in the IS in healthy T cells, implying a role for Kv1.3 in the termination of the Ca2+ response.
We conclude that altered localization of Kv1.3 channel in the IS of SLE T cells is at least in part responsible for more sustained Ca2+ response in SLE T cells. This phenotype, in turn, supports T cell hyperactivity in SLE T cells. Therefore we propose that Kv1.3 channels may offer novel therapeutic targets for SLE.