Cytotoxic CD8 T cells are highly motile lymphocytes that seek out and destroy specific targets in the immune response. Upon activation by antigen-binding, T cells aggregate together, forming clusters, but the functional significance of this clustering remains unclear.
Why do T cells aggregate? CD8 T cell activation involves recognition of an antigen displayed on MHC class I molecules by professional APCs. Interaction of CD8 T cell with Ag-bound APC is mediated by LFA-1, a molecule expressed on T cells that binds to ICAM-1 on APCs. The LFA-1/ICAM-1 adhesion facilitates T cell activation by promoting the joining of T cells and APCs, as well as initiating intracellular signaling that enhances TCR-mediated signals to promote T cell proliferation and differentiation. Because T cells express both LFA-1 and ICAM-1, the interaction between these two receptors also mediates aggregation of activated T cells! Studies have shown that this aggregation may regulate T cell proliferation and differentiation, but the functional significance of Ag-dependent clustering of T cells remains unknown.
A recent study conducted by Zumwalde et al. (2013) addressed activated T cell aggregation, and was described in Edition 191 of The Journal of Immunology. In this study, authors used an in vitro APC-independent CD8 T cell stimulation system to investigate the functional significance of aggregate formation during T cell activation. A line of ICAM-1-deficient mice were bred, and conjugated fluorescent Abs were used for labeling. Naïve eild-type and ICAM-deficient CD8 T cells were purified by negative selection. T cells were blocked in vitro from clustering using endotoxin-free anti-LFA-1 Abs. Flow cytometry and intracellular staining techniques as well as microscopy techniques were used to examine T cells.
Important experimental results of this study are as follows:
- The stimulation of purified naïve CD8 T cells resulted in enhanced expression of ICAM-1 on T cells, which was sustained by the inflammatory cytokine, IL-12.
- ICAM-1 deficient T cells proliferated normally but failed to aggregate following stimulation with Ag, B7-1, and IL-12. Compared to wild type T cells, ICAM-1-deficient cells displayed enhanced expression of both IFN-y and granzyme B, and enhanced cytotoxicity. Furthermore, inhibition of clustering of wild type T cells during activation with inhibitory anti-LFA-1 resulted in increased expression of IFN-y and granzyme B.
- Disruptions of T cell clustering was shown to enhance CD8 T cell effector functions by enhancing T cell ability to kill target cells compared with control wild type T cells. These results suggest that ICAM-1 mediated clustering negatively regulates CD8 T cell effector molecule production. Clustering of ICAM deficient CD8 T cells with wild type T cells reduces IFN-y expression
- A fluorescent reporter that marked TCR strength indicated that T cell clustering limits T cell exposure to Ag during activation. Furthermore, T cell clustering upregulates CTLA-4 inhibitory receptor and downregulates eomesodermin, which controls effector molecule expression.
- Activation of ICAM-1-deficient T cells enhanced the amount of KLRG-1 T cells, which are indicative of short-lived effecter molecules
These results show that T cell clustering represents a mechanism that allows continued proliferation while regulating T cell effector function and differentiation.
This study is interesting in that it suggests a functional reasoning for why T cells may aggregate after activation, an event that seems as though it would be either hindering to immune response or neutral to the immune response. When Ag-activated T cells begin to cluster, they have the potential of blocking capillaries and must be cleared, so it seems as though aggregation would hamper responses to pathogens.
One question that I still have after reading the article is, “How is negative regulation of CD8 T cell effector molecules advantageous to the immune response?”. A future study that could be conducted by the authors to follow up on this question would be to compare the downstream effects that take place in wild type CD8 T cells (reduction of IFN-y expression and reduced effector molecule production and cytotoxicity) in comparison to ICAM-deficient CD8 T cells (elevated expression of IFN-y and enhanced effector molecule production and cytotoxicity).
Zumwalde NA, E Domae, MF Mescher, and Y Shimizu. 2013. ICAM-1-Dependent Homotypic Aggregates Regulate CD8 T Cell Effector Function and Differentiation during T Cell Activation. Journal of Immunolgy, 191: 3681-3693
von Andrian, U. H., and T. R. Mempel. 2003. Homing and cellular traffic in lymph nodes. Nat. Rev. Immunol. 3: 867–878