Mechanisms controlling granule-mediated cytolytic activity of cytotoxic T lymphocytes

Abstract

Cytotoxic T lymphocytes (CTL) play a critical role in immunity against viruses and cancer. The antigen receptor or T-cell receptor (TCR) on CTL determines the specificity toward target cells. The CD8 co-receptor functions in concert with the TCR to enhance TCR-mediated signaling, accounting for the remarkable sensitivity and swift signaling kinetics of the CTL response. The latter ensures efficient delivery and release of lytic granules, resulting in sensitive and rapid destruction of target cells.

Figure 1

Changes of cellular morphology and the structure of the synaptic interface in CTL stimulated with supported bilayers that display various combinations of cognate pMHC and ICAM-1 ligands. Upper left: CTL exposed to ICAM-1-containing bilayers in the absence of cognate pMHC still remodel cytoskeleton as evident from the dramatic increase in area of the contact membrane and the formation of the adhesion ring junction. CD8 and TCR are accumulated within the ring junction albeit to a lesser extent. CTL adhesion is transient and individual cells undergo several cycles of detachment and subsequent adhesion to the bilayers. Bottom left: CTL recognizing cognate pMHC at high density in the absence of LFA-1 ligand (ICAM-1) adhered to the bilayers through a very small area. There is no accumulation of TCR at the adhesion area and the CTL sporadically oscillated around this area. The CTL remained attached to the bilayers for a long time. Upper right: After initial contact with bilayers containing cognate pMHC and ICAM-1, CTL adhere to the bilayer, showing T-cell spreading and rapid accumulation of pMHC and ICAM-1 at distinct locations over the interface (a nascent CS). Bottom left: The foci containing aggregated pMHC and ICAM-1 then coalesced followed by large-scale molecular segregation and the formation of the cSMAC and pSMAC (a mature CS).

Figure 2

Two potential pathways of cytolytic granule delivery to the CS are associated with different early TCR signaling kinetics and target lysis kinetics by CTL. Upon antigen stimulation, more (CD8+) or less (CD4+) effective CTL polarize their granules to either the center (a) or periphery (b) of the CS, respectively. We propose that cytolytic granules are rapidly concentrated around the MTOC prior to MTOC polarization in CD8+ CTL (c), but not in CD4+ CTL (d), explaining their topography within the CS in these two CTLs. Consistent with this, initial TCR engagement in more effective CD8⁺ CTL results in rapid and robust early TCR signaling (e) as opposed to weak early signaling induced in less effective CD4⁺ CTL (f). As a result, cytolytic granules are delivered and released at the CS more rapidly in more effective CTL, resulting in faster kinetics of target cell destruction by CD8+ (g), but not by CD4+ (h) CTL.