2.1. T cells and where to find them
In physiological conditions, the highest concentration of immune cells in the CNS can be found within the dura mater. These include T and B lymphocytes, macrophages and other myeloid cells (Buckley & McGavern, 2022; Louveau et al., 2015). T lymphocytes travel from CNS postcapillary venules to the meninges, specifically to dura mater and the SAS (Mrdjen et al., 2018; Ransohoff & Engelhardt, 2012). T cells exit via lymphatic vasculature and reach the draining cervical lymph nodes (Buckley & McGavern, 2022). Although the skull and vertebral bodies possess bone marrow pockets (Cugurra et al., 2021), the majority of T lymphocytes in the CNS comes from the blood (Rustenhoven et al., 2021).
The process of diapedesis requires several steps, starting with the capture of immune cells on the endothelium by selectins, which slows down their movement. The recognition of their G-protein coupled receptors (GPCRs) then allows the arrest and crawling of immune cells in an integrin-dependent manner, leading to the passage through the endothelium (Mapunda, Tibar, Regragui & Engelhardt, 2022). T lymphocytes diapedesis from CNS postcapillary venules is restricted to activated memory T cells, which are capable of expressing C-C chemokine receptor type 7 (CCR7) and recognizing C-C chemokine ligand type 19 (CCL19), which facilitates the migration of this subset of T cells through the BBB (Marchetti & Engelhardt, 2020). Moreover, the BBB endothelial cells constitutively express atypical chemokine receptor 1 (ACKR1), which, by transporting chemokines to the luminal side of the venular endothelium, restricts the crossing of this barrier to activated T cells (Marchetti et al., 2022). In steady state, CD4+ T lymphocytes are able to cross the BBB through leukocyte function-associated antigen-1 (LFA-1)-dependent adhesive interactions (Castro Dias et al., 2021; Marchetti & Engelhardt, 2020). Different types of T helper cells may use different mechanisms and sites to perform the diapedesis process (Mundt, Greter, Flugel & Becher, 2019). For instance, T helper 17 (Th17) lymphocytes, which express CCR6 and CCR4, preferably enter via CP-associated blood-CP barrier (BCPB) by LFA-1. On the other hand, T helper 1 (Th1) pass the BBB in the spinal cord microvessels through VLA-4 (Glatigny, Duhen, Oukka & Bettelli, 2011; Rothhammer et al., 2011). In addition, CD8+ T cells are more dependent on LFA-1 for diapedesis in the CNS and also require endothelial junctional adhesion molecule B (JAM-B) to pass through the BBB (Alvarez et al., 2015; Mapunda, Tibar, Regragui & Engelhardt, 2022).
The repertoire of immune cells in a non-neuroinflammatory state has been characterized by many authors. Analysis using single-cells techniques has revealed a wide variety of immune populations, such as border-associated macrophages (BAMs), monocytes, natural killer (NK) cells, dendritic cells, innate lymphoid cells, B and T lymphocytes. In the healthy CNS, T cells could be found patrolling the meninges and the CP, but not in the brain parenchyma (Table 1) (Mundt, Greter, Flugel & Becher, 2019; Prinz & Priller, 2017). Due to the restrictions of the BBB, activated T cells with a memory phenotype and fewer tissue-resident antigen-presenting cells (APCs) can be found in the dura mater, as well as the SAS and CSF (de Graaf et al., 2011; Kawakami & Flugel, 2010; Kivisakk, Tucky, Wei, Campbell & Ransohoff, 2006). Moreover, these T lymphocytes upregulate genes and cytotoxicity-related markers that resemble tissue-resident T cells (Croese, Castellani & Schwartz, 2021; Norris & Kipnis, 2019). T cells constitute a significant portion of the immune environment within the CSF, making up as much as 84% of the total of immune cells. Among T cells, CD4+ cells comprise 48.95%, CD8+ cells represent 17.61%, double positive CD4+CD8+ cells represent 9.63%, regulatory T cells (Treg) constitute 2.49%, and natural killer (NK) cells make up 3.45%. The ratio of CD4+ T cells to CD8+ T cells is estimated to be around 3.1 to 1. (Piehl et al., 2022; Ransohoff & Engelhardt, 2012). .