Role of IL-16 in CD4+ T cell-mediated regulation of relapsing multiple sclerosis

In an important article published in Nature Medicine, Liu and colleagues described a novel CD4(+) FoxA1(+) regulatory T (Treg) cell population as distinct regulators of relapsing-remitting multiple sclerosis (RRMS) and experimental autoimmune encephalomyelitis (EAE). CD4(+) FoxA1(+) Treg cells appear as key regulators of responsiveness to therapy with interferon beta (IFN-β) in RRMS patients. Data indicate that CD4(+)FoxA1(+) FOXP3(-) Treg cells develop within the central nervous system (CNS), and a potential of cerebellar granule neurons (CGN) in generation of CD4(+)FoxA1(+)PD-L1(hi)FOXP3(-) Treg cells from encephalitogenic CD4(+) T cells. A CD4 co-receptor specific ligand, IL-16, governs trafficking and biological properties of CD4(+) T cells irrespective of their activation state. Functions of IL-16, relevant to Treg cells, include expansion of CD4(+)CD25(+) T cells in long-term cultures with IL-2, de novo induction of FOXP-3 and migration of FOXP-3(+) T cells. IL-16 is highly conserved across species including human and mouse. CGN and neurons in hippocampus contain neuronal-IL-16 (NIL-16), splice variant of immune IL-16, and express CD4 molecule. In a CD4-dependent manner, IL-16 supports cultured CGN survival. Concomitant studies of RRMS lesions and corresponding MOG35-55-induced relapsing EAE in (B6 × JL)F1 (H-2(b/s)) mice discovered similar roles of IL-16 in regulation of relapsing disease. In RRMS and EAE relapse, peak levels of IL-16 and active caspase-3 correlated with CD4(+) T cell infiltration and levels of T-bet, Stat-1(Tyr(701)), and phosphorylated neurofilaments of axonal cytoskeleton [NF (M + H) P], suggesting a role of locally produced IL-16 in regulation of CD4(+) Th1 inflammation and axonal damage, respectively. IL-16 was abundantly present in CD4(+) T cells, followed by CD20(+) B, CD8(+) T, CD83(+) dendritic cells, and Mac-1(+) microglia. Apart from lesions, bioactive IL-16 was located in normal-appearing white matter (NAWM) and normal-appearing grey matter (NAGM) in RRMS brain and spinal cord. A cytokine IL-16 emerges as an important regulator of relapsing MS and EAE. Better understanding of immune cell-neuron interactions mediated by IL-16 will foster development of more specific CD4(+) T cell subset-targeted therapies to prevent or ameliorate progression of neuroinflammation and axonal and neuronal damage. Translational studies necessitate corresponding EAE models.