Multiple sclerosis (MS) is a chronic autoimmune disease of the central nervous system (CNS) in which myelin-specific T lymphocytes migrate across an impaired blood-brain barrier (BBB) and cause an inflammatory and demyelinating pathology. Although T lymphocytes are traditionally divided into CD4+ helper and CD8+ cytotoxic T lymphocytes (CTLs), CD4+ T cells can become cytotoxic during chronic inflammation. Recently, the activation marker 'class I-restricted T cell-associated molecule' (CRTAM) was identified as a key molecule in the formation of CD4+ CTLs in mice. CRTAM interacts with its ligand nectin-like 2 (Necl-2), which promotes the cytotoxic function and adhesive properties of activated NK cells and CD8+ CTLs. However, the contribution of CD4+CRTAM+ T cells to MS disease pathology has not been investigated yet. In this study, we identified CD4+CRTAM+ T cells in MS brain lesions, which supports their tissue-infiltrating capacity. In addition Necl-2 is highly expressed by BBB-associated astrocytes. Next, we examined the role of CD4+CRTAM+ T cells in neuroinflammation in vivo. Therefore, experimental autoimmune encephalomyelitis (EAE) was induced in wild-type and CRTAM-transgenic mice (CRTAM-Tg/Lck), displaying a transient T cell-specific CRTAM overexpression after activation. Surprisingly, only 25% of CRTAM-Tg mice developed EAE-related paralysis symptoms, while the symptom-free CRTAM-Tg mice acquired anxiety-like behaviour, evaluated using the open field test. At the time of disease onset, flow cytometric analysis revealed that CD4+ effector T cells did expand in the draining lymph nodes, but did not accumulate in the CNS of CRTAM-Tg mice. This was accompanied by reduced BBB leakage in the spinal cord and surprisingly, increased reactive astrocytes in the cortex. The latter could explain the altered behavioural changes. Together, these data suggest that autoreactive CRTAM-Tg CD4+ T cells are unable to cross the BBB and cause CNS damage after EAE induction. We hypothesize that this is attributable to CRTAM-Necl-2 interaction at the level of the glia limitans, lined by Necl-2+ astrocytes, which triggers an unexpected physiological response leading to astrogliosis and, consequently, BBB closure. Additional in vitro experiments will test this new hypothesis.
