SLFN2 protection of tRNAs from stress-induced cleavage is essential for T cell-mediated immunity.

INTRODUCTION Naive T cells activated by cognate antigens and costimulation proliferate and differentiate to effector T cells. The shift from a resting to a proliferative state entails profound changes in cellular metabolism, in particular increases in glycolysis, glutaminolysis, and mitochondrial metabolism, to produce high levels of adenosine 5′-triphosphate (ATP). T cells depend on a translational burst to produce the metabolic enzymes that support an increase in metabolism and to produce the protein components of clonal T cell progeny and their cytokines. Paradoxically, the metabolic processes that provide energy for growth and expansion also produce reactive oxygen species (ROS). These are capable of inducing oxidative stress, which leads to the repression of translation. On the other hand, ROS function as second messengers in T cell receptor (TCR) signaling and are essential for proliferation and development of effector function. This suggests that to preserve the signaling activities of ROS, protective mechanisms against oxidative stress may occur at multiple levels beyond simply reducing ROS levels in T cells. RATIONALE From a mouse-forward genetic screen for mutations affecting immunity, we previously identified a recessive mutation in the Schlafen 2 (Slfn2) gene, which leads to elevated susceptibility to bacterial and viral infections and to diminished numbers of T cells that failed to proliferate in response to infection and diverse proliferative stimuli. Here, we aimed to investigate the molecular function of SLFN2 in T cells by generating mice with a T cell–specific deletion of Slfn2. RESULTS T cell–specific SLFN2-deficient mice displayed compromised humoral and cellular immune responses to immunization with a T cell–dependent antigen and to infection with mouse cytomegalovirus, respectively. These defects stemmed from impaired CD4+ and CD8+ T cell proliferative responses to TCR stimulation, despite normal induction of TCR signaling events in SLFN2-deficient T cells. Interleukin-2 (IL-2) production by SLFN2-deficient T cells was normal after TCR stimulation, but these cells failed to proliferate in response to exogenous IL-2, which suggests that interleukin-2 receptor (IL-2R) signaling was defective. The abrogation of the mitogenic effects of IL-2 was a result of a failure to translationally up-regulate the β and γ chains of the IL-2 receptor. There was a globally dampened translational response to TCR activation in SLFN2-deficient T cells both in vitro and in vivo. The cellular oxidative stress response includes translation repression by transfer RNA (tRNA) fragments generated by angiogenin (ANG), a stress-induced tRNA-directed ribonuclease (RNase). ANG cleaves tRNAs within their anticodon loops, yielding 30- to 40-nucleotide tRNA fragments (tiRNAs). In response to TCR activation, SLFN2-deficient T cells accumulated tiRNA, which could be reduced by antioxidant treatment or by knockdown or inhibition of ANG. Moreover, global translation rates in activated SLFN2-deficient T cells could be rescued by antioxidant treatment or by ANG knockdown. SLFN2 directly bound to tRNAs, but it exerted no nucleolytic activity toward them, unlike other SLFN proteins. Binding of SLFN2 to tRNAs blocked tRNA cleavage by ANG, thereby averting tiRNA accumulation and tiRNA-mediated translation repression. CONCLUSION We describe a protective mechanism by which SLFN2 shields tRNA from oxidative stress–induced cleavage, thereby preventing the translation inhibitory effects of ROS produced in response to T cell activation. Notably, SLFN2 acts downstream of ROS production itself, leaving ROS functions in T cell metabolism and signaling intact. We identify ANG as a stress-activated RNase whose effects are opposed by SLFN2 in T cells. Our data provide further support for a key role of SLFN family members in the regulation of RNA and translation.