The DNA Sensor AIM2 Determines B Cell Fate after BCR Activation in Chronic Graft-Vs-Host Disease
Authors: Basher Fahmin , Visentin Jonathan , Jia Wei , Poe Jonathan C , Sofian Christina , Su Hsuan , Bracken Sonali , Dicioccio Rachel , Curry-Chisolm Itaevia , Ho Vincent T. , Sisirak Vanja , Horwitz Mitchell , Chao Nelson J. , Sarantopoulos Stefanie 
Published: December 2023

Prevention and treatment of chronic graft-vs-host disease (cGVHD), a debilitating late immune toxicity in patients after allogeneic hematopoietic stem cell transplantation (allo-HCT), who are otherwise cured of hematological malignancies, remains a major challenge. T cells incite and promote cGVHD through coordinated T-B cell responses. Alloantibody production ( Srinivasan et al, Blood 2012, Jin et al, Blood 2016) and aberrant B Cell Receptor (BCR) signaling ( Allen et al, Blood 2014, Flynn et al, Blood 2015) play fundamental roles in the pathology of cGVHD. B cell-intrinsic pathways in cGVHD remain incompletely understood, limiting our therapeutic capacity for patients.
In a single cell RNA sequencing study, we found that transcript levels of the DNA sensor Absent in Melanoma 2 (AIM2) were significantly increased in CD27+ BCR-activated B cell clusters in circulating as well as in tissue-resident cGVHD B cells ( Poe et al, JCI Insight 2023). After confirming AIM2 expression is restricted to the nucleus, we also previously showed that AIM2-expressing B cells were augmented after BCR and interferon-gamma (IFNγ) and/or Toll-like receptor (TLR)-7 or 9 stimulation ( Visentin J, Blood 2022;140 Supp 1), suggesting a context-dependent role for AIM2 after allo-HCT.
Our new findings now afford the hypothesis that an AIM2-BCR signaling axis determines B cell fate after allo-HCT. To address this hypothesis, we first examined whether AIM2 modulates the promotion of antibody-secreting cells (ASCs) after BCR activation using a hapten-carrier immunization model. We compared numbers of hapten-specific ASCs from immunized WT and AIM2-KO mouse splenocytes using enzyme-linked immunosorbent spot (ELISPOT). We compared numbers of ASCs on day 3 post-immunization, when early germinal center (GC) formation is known to occur. We found that hapten-specific IgM ASCs were significantly increased in WT splenocytes compared to AIM2-KO (p=0.002). By contrast, when we looked at day 8, when mature GC formation and rapid ASC proliferation is known to occur, we found that AIM2-KO immunized mice had significantly lower numbers of hapten-specific IgM (p=0.005) and IgG (p=0.004) ASCs in AIM2-expressing mice (Figure 1a). Our results suggest a role for AIM2 in regulating the survival of short-lived plasmablast-like ASCs after high affinity antigen encounter and maturation of germinal centers.
To further investigate whether AIM2 affects expansion of GC-derived (GL7+CD95+) ASCs, we stimulated B cells purified from wildtype (WT) or global AIM2-deficient (AIM2-KO) mice in vitro with lipopolysaccharide (LPS) and IL-4 +/- BCR stimulation. We found that AIM2-deficient B cells, especially GC-like cell populations, expanded significantly in an anti-IgM concentration-dependent manner up to 12-fold (p=0.03) after 72 hr (Figure 1b). By contrast, B cell expansion was constrained in AIM2-sufficient B cells. This expansion was unlikely to be due to differences in proliferation alone, as we did not detect differential Ki67 expression in stimulated B cells with or without AIM2. Together, these data suggest a role for AIM2 in modulating BCR signaling.
Finally, we examined the role of AIM2 in BCR-mediated cell death after chronic antigen exposure using our previously described bone marrow transplant (BMT) model, where recipients of bone marrow and splenocytes (BM+Spl) exhibit cGVHD manifestations ( Poe et al, JCI Insight 2018, Jia et al, Blood 2021). We have previously generated C57BL/6J mice with conditional deficiency of AIM2 only in B cells (AIM2 fl/flMb1 Cre/+, CKO), and used these in our BMT model as donors. At day 58 post BMT, we performed ex vivo stimulation of splenic B cells from BMT recipients. While B cells from CKO BM+Spl recipients expanded ex vivo in an anti-IgM concentration-dependent manner, B cells from AIM2-sufficient BM+Spl recipients demonstrated limited responsiveness ex vivo to BCR stimulation. This extends our in vitro observations by suggesting a role for AIM2 in B cell tolerance after chronic antigen exposure. Ongoing experiments will determine if B cell-intrinsic AIM2 contributes to cGVHD pathogenesis.
Thus far, our data reveal a novel role for AIM2 as a B cell survival checkpoint after BCR activation. Further delineation of context-dependent and synergistic AIM2-BCR signaling after allo-HCT will lead to strategies that block the development of cGVHD without adversely affecting anti-tumor activity.