Acute graft‐versus‐host disease (GvHD) is a serious complication after allogeneic bone marrow transplantation. Donor‐derived T cells infiltrate recipient target organs and cause severe tissue damage, often leading to death of the affected patient. Tissue destruction is a direct result of donor CD8⁺ T cell activation and cell‐mediated cytotoxicity. IL‐18 is a novel pro‐inflammatory cytokine with potent T_h1 immune response‐promoting and cytotoxic T lymphocyte (CTL)‐inducing activity. IL‐18 is strongly induced in experimental mouse models and human patients with acute GvHD. However, the precise role of IL‐18 in the development of acute GvHD is still unknown. In this study, we have used IL‐18‐binding protein, a soluble IL‐18 decoy receptor, to specifically neutralize IL‐18 in vivo and in vitro. Our results demonstrate that IL‐18 is induced during GvHD. However, its effect in the induction of GvHD appears to be redundant, since neutralization of IL‐18 does not alter any disease parameter analyzed. Our study further shows that IFN‐γ production and CTL induction upon activation by T cell mitogens or by alloantigen does not involve IL‐18‐mediated amplification, in contrast to lipopolysaccharide‐induced IFN‐γ production. We conclude that IL‐18 expression correlates with the course of GvHD; however, its effect is dispensable for IFN‐γ and CTL induction for the initiation phase of this disease, most likely due to direct, IL‐18‐independent, CTL activation.

Multiple sclerosis (MS) is a chronic, demyelinating disease of the central nervous system (CNS) induced by immune dysregulation. Cladribine has been championed for its clinical efficacy with relatively minor side effects in treating MS. Although it is proposed that cladribine exerts an anti-migratory effect on lymphocytes at the blood–brain barrier (BBB) in addition to its lymphocyte-depleting and modulating effects, this has not been properly studied. Here, we aimed to determine if cladribine treatment influences trans-endothelial migration of T cell subsets across an inflamed BBB. Human brain endothelial cells stimulated with pro-inflammatory cytokines were used to mimic the BBB. Peripheral blood mononuclear cells were obtained from healthy controls, untreated and cladribine-treated MS patients. The trans-endothelial migration of CD4⁺ effector memory T (T^EM) and CD8⁺ central memory T (T^CM) cells was reduced in cladribine-treated MS patients. CD28 expression was decreased on both CD4⁺ T^EM and CD8⁺ T^CM cells, suggesting lowered peripheral activation of these cells thereby maintaining the integrity of the BBB. In addition, these cells have likely reconstituted following cladribine treatment, revealing a long-term anti-migratory effect. These results highlight new mechanisms by which cladribine acts to control MS pathogenesis.

The breakdown of the blood–brain barrier (BBB) and the trans-endothelial migration of lymphocytes are central events in the development of multiple sclerosis (MS). Autoreactive T cells are major players in MS pathogenesis, which are rapidly depleted following alemtuzumab treatment. This modulation, in turn, inhibits CNS inflammation, but alemtuzumab’s effect on T cell migration into the CNS has been less studied. Human brain endothelial cells were stimulated with pro-inflammatory cytokines to mimic an inflamed BBB in vitro. Peripheral blood mononuclear cells from healthy controls, untreated or alemtuzumab-treated patients with relapsing-remitting MS (RRMS) were added to the BBB model to assess their transmigratory capacity. Here, the migration of CD4⁺ effector memory T (T^EM) and CD8⁺ central memory T (T^CM) cells across the BBB was impaired in alemtuzumab-treated patients. Naïve T (T^naïve) cells were unable to migrate across all groups. CD38 was lowly expressed on CD8⁺ T^CM cells, particularly for RRMS patients, compared to CD8⁺ T^naïve cells. CD62L expression was lower on CD4⁺ T^EM cells than CD4⁺ T^naïve cells and decreased further in alemtuzumab-treated patients. These data suggest that repopulated memory T cells are phenotypically different from naïve T cells, which may affect their transmigration across the BBB in vitro.

Multiple sclerosis (MS) is a disease in which the immune system damages components of the central nervous system (CNS), leading to the destruction of myelin and the formation of demyelinating plaques. This often occurs in episodic “attacks” precipitated by the transmigration of leukocytes across the blood-brain barrier (BBB), and repeated episodes of demyelination lead to substantial losses of axons within and removed from plaques, ultimately leading to progressive neurological dysfunction. Within leukocyte populations, macrophages and T and B lymphocytes are the predominant effectors. Among current immunotherapies, oral cladribine’s impact on lymphocytes is well characterised, but little is known about its impact on other leukocytes such as monocytes and dendritic cells (DCs). The aim of this study was to determine the transmigratory ability of monocyte and DC subsets in healthy subjects and untreated and cladribine-treated relapse-remitting MS (RRMS) patients using a well-characterised model of the BBB. Peripheral blood mononuclear cells from subjects were added to an in vitro transmigration assay to assess cell migration. Our findings show that while prior treatment with oral cladribine inhibits the migration of intermediate monocytes, it has no impact on the transmigration of DC subsets. Overall, our data indicate a previously unrecognised role of cladribine on intermediate monocytes, known to accumulate in the brain active MS lesions.