The BCL-2 pro-survival protein A1 is dispensable for T cell homeostasis on viral infection

Abstract

The physiological role of the pro-survival BCL-2 family member A1 has been debated for a long time. Strong mRNA induction in T cells on T cell receptor (TCR)-engagement suggested a major role of A1 in the survival of activated T cells. However, the investigation of the physiological roles of A1 was complicated by the quadruplication of the A1 gene locus in mice, making A1 gene targeting very difficult. Here, we used the recently generated A1−/− mouse model to examine the role of A1 in T cell immunity. We confirmed rapid and strong induction of A1 protein in response to TCR/CD3 stimulation in CD4 + as well as CD8 + T cells. Surprisingly, on infection with the acute influenza HKx31 or the lymphocytic choriomeningitis virus docile strains mice lacking A1 did not show any impairment in the expansion, survival, or effector function of cytotoxic T cells. Furthermore, the ability of A1 −/− mice to generate antigen-specific memory T cells or to provide adequate CD4-dependent help to B cells was not impaired. These results suggest functional redundancy of A1 with other pro-survival BCL-2 family members in the control of T celldependent immune responses. Cell Death and Differentiation (2017) 24, 523-533; doi:10.1038/cdd.2016.155; published online 13 January 2017On antigenic challenge, T lymphocytes need to rapidly switch from their IL-7/IL-7R-regulated naive, quiescent state 1,2 to a T cell antigen-receptor (TCR/CD3) stimulation-induced activation state. 3 In case of inappropriate stimulation of the TCR, for example, in the absence of co-receptor stimulation, this shift in the survival programme is not induced and leads to rapid T cell death. 4 Conversely, appropriately stimulated T cells expand rapidly, allowing accumulation of T cell clones expressing TCRs of high affinity for specific antigens. During this clonal expansion, BCL-2 family regulated apoptosis acts as a mechanism to remove low-affinity T cells, thereby ensuring the generation of a highly effective immune response.5 On infection clearance, most of the activated T cells are removed by apoptosis, 6 leaving only some T cells with antigen-specific high-affinity TCRs in reserve as longlived memory T cells. 7The BCL-2 family of proteins regulate apoptotic cell death, with the balance between pro-survival and pro-apoptotic family members determining whether a cell lives or dies. The expression of pro-survival BCL-2 family members is dynamically regulated during T cell activation.8 TCR/CD3 ligation leads to the downregulation of BCL-2 and induction of BCL-XL.3 Accordingly, Bcl-2 −/− mice display a loss of mature, unstimulated T cells, and the death of these cells can be prevented by TCR/CD3 stimulation.9 Interestingly, although BCL-XL is substantially upregulated on TCR/CD3 stimulation, its loss did not increase apoptosis or impair proliferation of T cells stimulated with mitogenic antibodies. 10 In contrast, MCL-1 has been shown to be a crucial pro-survival factor after T cell activation.11,12 A1 is a prosurvival BCL-2 family protein that has been proposed to be i...

Characterisation of mice lacking all functional isoforms of the pro-survival BCL-2 family member A1 reveals minor defects in the haematopoietic compartment

“…The in vivo T cell stimulation has been examined using models of influenza and chronic LCMV infection, but no abnormalities were seen in A1 −/− mice. 44 Furthermore, upon immunisation with NP-KLH, A1 −/− mice failed to reveal any B cell activation defects in vivo. 44 It has been reported that in mice lacking the A1-a isoform, the inflammatory response to infection with Toxoplasma gondii was less severe compared with wild-type controls in terms of overall peritoneal leukocyte cellularity, but the proportions of granulocytes recruited to the peritoneal cavity were comparable.…”

“…44 Furthermore, upon immunisation with NP-KLH, A1 −/− mice failed to reveal any B cell activation defects in vivo. 44 It has been reported that in mice lacking the A1-a isoform, the inflammatory response to infection with Toxoplasma gondii was less severe compared with wild-type controls in terms of overall peritoneal leukocyte cellularity, but the proportions of granulocytes recruited to the peritoneal cavity were comparable. 45 This suggests that loss of A1 may impair the survival of certain cell types of the immune system to a minor extent, but not enough to preclude A1-deficient mice to respond adequately to infectious challenges.…”

Dying to protect: cell death and the control of T‐cell homeostasis

“…However, recent work examining a complete A1 KO mouse showed no decrease in the overall numbers of naïve T cells (225). In activated T cells, A1 is mainly thought to be critical for activated T cell survival due to its rapid response to TCR signaling and the fact that it does not inhibit T cell proliferation (208, 209).…”

“…Preliminary data from our lab shows higher A1 expression in CD4 effector cells compared to CD8 effectors at the peak of an anti-viral response, suggesting a CD4-specific role for A1 in effector T cell survival. Although the recent work in the complete A1 KO suggested that effector T cell responses were normal, it should be noted that this work did not examine the CD4 + T cell response using MHC tetramers, but instead just looked at overall numbers of memory cells after infection (225). In addition, the mice did display a significant loss of memory CD4 + T cells, suggesting that either the contraction of the effector response or the maintenance of the memory CD4 compartment relies substantially on A1.…”

Differential effects of Vav‐promoter‐driven overexpression of BCLX and BFL1 on lymphocyte survival and B cell lymphomagenesis

“…In mice, A1 is produced by three independent genes (Bcl2a1-a, Bcl2a1-b and Bcl2a1-d) [5] and is mainly expressed in the haematopoietic system where it is dynamically regulated in response to antigens or inflammatory cues engaging NF-kB [6], NF-AT [7], and PU.1 [8] transcription factors. Mice that lack all functional Bcl2a1 genes do not exhibit major impairments in the development and composition of their immune system [9] or T cell-mediated immune responses [10]. The human homologue, BFL1, which is highly homologous to A1 (72% amino acid identity) is encoded by a single gene [11].…”