T cell surveillance of the CNS

Phenotyping T cells in human post mortem brain

Joost Smolders, MD, PhD

The central nervous system (CNS) is a structurally and functionally unique yet vulnerable organ that requires efficient immune protection against viral infections. It has become clear that most tissues are populated by tissue-resident memory T cells (T_RM), which are adapted to their niche and appear to be indispensable for local protection against infection. We previously showed that the human brain is surveyed by a specific pool of CD8⁺, and to a lesser extent CD4⁺, T cells with an effector-type phenotype, limited expression of cytolytic enzymes, and high expression of the IL-7 receptor (CD127), indicating maintenance by homeostatic proliferation (Smolders et al, Acta Neuropathol 2013; 126:525-35). Whether these cells represent T_RM cells has remained unknown.

We currently characterize the phenotypical and functional characteristics of human brain CD8⁺ T cells, by assessing differentiation status, transcription factor profiles, chemokine receptor profiles, and expression of cytokines and lytic enzymes by flow cytometry. Our data indicate that brain CD8⁺ T cells are represented by two subsets of CD69⁺CD103⁻ and CD69⁺CD103⁺ T cells. CD103 expression in brain CD8⁺ T_RM cells correlates with enhanced differentiation, parenchymal localization, and restricted cytotoxic capacity, marking them as T_RM cells.

t-SNE plot of paired CD8⁺ T cells from brain (red) and blood (blue) based on expression of markers shown in this figure. Expression of individual markers was analyzed by conventional gating and by assessment of the distribution within the t-SNE plot. Brain CD8⁺ T cells are most prominently characterized by high expression of CD69 and CD103. The low expression of CD27, CD28, CD45RA, and CCR7 in combination with high expression of CD45RO and CD127 reflects an effector-memory phenotype.