The curative potential of allogeneic haematopoietic stem cell transplantation (allo-HSCT) relies strongly upon the immune responses against host antigens mediated by donor T lymphocytes as effectors of anti-tumour immune surveillance [1]. The graft-versus-leukaemia (GVL) effect is exploited further by the use of delayed infusions of donor lymphocytes (DLIs) [2]. However, the therapeutic impact of donor lymphocytes is limited by the risk of allogeneic acute graft-versus-host Akt cancer disease

(aGVHD). Approximately 55–60% of patients treated with bulk doses of DLIs for recurrent leukaemia develop aGVHD [3]. Recent research has demonstrated that naive but not memory donor T cells are capable of inducing aGVHD [4,5]. aGVHD requires the stimulation of naive donor T cells by antigen-presenting cells (APC). Residual host APCs alone are sufficient for the induction of CD8+ T cell-dependent aGVHD [6]. Following cognate interaction with activated APC, CD4+ T cells are driven towards T helper type 1 (Th1)-biased cytokine production [7], promoting T cell proliferation and further differentiation, so

that very large amounts of proinflammatory cytokines are generated which induce tissue XL184 mw damage in a major histocompatibility complex (MHC)-independent fashion [8]. It has been shown that the infusion of purified CD4+ T cells as DLI resulted in the expansion of CD8+ T cells, suggesting the critical importance of CD4+ T cells in regulating the expansion of CD8+ T cells which mediate aGVHD [9], and the differentiation of CD4+

T cells into Th1 is dictated by the nature of the donor T cell–host APC interaction [10–12]. 17-DMAG (Alvespimycin) HCl Therefore, Th1 cells not only mediate aGVHD, but also play a critical role in amplifying aGVHD. We hypothesized that inhibiting artificially the Th1-type differentiation of donor naive CD4+ T cells could prevent aGVHD. Suppressor of cytokine signalling (SOCS) proteins comprise a family of intracellular regulators of cytokine-induced signal transduction. SOCS protein expression is inducible by cytokines, and once expressed, SOCS proteins inhibit cytokine signalling by switching off the Janus kinase/signal transducer and activator of transcription (JAK/STAT) pathway [13,14]. SOCS expression has been observed at many stages of T cell development and function, and it has been suggested that SOCS-1 and SOCS-3 are important regulators of T cell activation, differentiation and homeostasis [15–19]. It has been shown that naive CD4+ T cells constitutively express low levels of SOCS-1 and SOCS-3 mRNA [15]. Differentiation into Th1 or Th2 phenotypes is accompanied by preferential expression of distinct SOCS mRNA transcripts and proteins. SOCS-1 expression is fivefold higher in Th1 cells than in Th2 cells, whereas Th2 cells contain 23-fold higher levels of SOCS-3.