IL-33 acts as a costimulatory signal to generate alloreactive Th1 cells in graft-versus-host disease

Antigen-presenting cells (APCs) integrate signals emanating from local pathology and program appropriate T cell responses. In allogeneic hematopoietic stem cell transplantation (alloHCT), recipient conditioning releases damage-associated molecular patterns (DAMPs) that generate proinflammatory APCs that secrete IL-12, which is a driver of donor Th1 responses, causing graft-versus-host disease (GVHD). Nevertheless, other mechanisms exist to initiate alloreactive T cell responses, as recipients with disrupted DAMP signaling or lacking IL-12 develop GVHD. We established that tissue damage signals are perceived directly by donor CD4⁺ T cells and promoted T cell expansion and differentiation. Specifically, the fibroblastic reticular cell–derived DAMP IL-33 is increased by recipient conditioning and is critical for the initial activation, proliferation, and differentiation of alloreactive Th1 cells. IL-33 stimulation of CD4⁺ T cells was not required for lymphopenia-induced expansion, however. IL-33 promoted IL-12–independent expression of Tbet and generation of Th1 cells that infiltrated GVHD target tissues. Mechanistically, IL-33 augmented CD4⁺ T cell TCR-associated signaling pathways in response to alloantigen. This enhanced T cell expansion and Th1 polarization, but inhibited the expression of regulatory molecules such as IL-10 and Foxp3. These data establish an unappreciated role for IL-33 as a costimulatory signal for donor Th1 generation after alloHCT.

Graft IL-33 regulates infiltrating macrophages to protect against chronic rejection

Alarmins, sequestered self-molecules containing damage-associated molecular patterns, are released during tissue injury to drive innate immune cell proinflammatory responses. Whether endogenous negative regulators controlling early immune responses are also released at the site of injury is poorly understood. Herein, we establish that the stromal cell-derived alarmin interleukin 33 (IL-33) is a local factor that directly restricts the proinflammatory capacity of graft-infiltrating macrophages early after transplantation. By assessing heart transplant recipient samples and using a mouse heart transplant model, we establish that IL-33 is upregulated in allografts to limit chronic rejection. Mouse cardiac transplants lacking IL-33 displayed dramatically accelerated vascular occlusion and subsequent fibrosis, which was not due to altered systemic immune responses. Instead, a lack of graft IL-33 caused local augmentation of proinflammatory iNOS+ macrophages that accelerated graft loss. IL-33 facilitated a metabolic program in macrophages associated with reparative and regulatory functions, and local delivery of IL-33 prevented the chronic rejection of IL-33-deficient cardiac transplants. Therefore, IL-33 represents what we believe is a novel regulatory alarmin in transplantation that limits chronic rejection by restraining the local activation of proinflammatory macrophages. The local delivery of IL-33 in extracellular matrix-based materials may be a promising biologic for chronic rejection prophylaxis.

PIRs mediate innate myeloid cell memory to nonself MHC molecules

Immunological memory specific to previously encountered antigens is a cardinal feature of adaptive lymphoid cells. However, it is unknown whether innate myeloid cells retain memory of prior antigenic stimulation and respond to it more vigorously on subsequent encounters. In this work, we show that murine monocytes and macrophages acquire memory specific to major histocompatibility complex I (MHC-I) antigens, and we identify A-type paired immunoglobulin-like receptors (PIR-As) as the MHC-I receptors necessary for the memory response. We demonstrate that deleting PIR-A in the recipient or blocking PIR-A binding to donor MHC-I molecules blocks memory and attenuates kidney and heart allograft rejection. Thus, innate myeloid cells acquire alloantigen-specific memory that can be targeted to improve transplant outcomes.

IL-33-mediated IL-13 secretion by ST2+ Tregs controls inflammation after lung injury

Acute respiratory distress syndrome is an often fatal disease that develops after acute lung injury and trauma. How released tissue damage signals, or alarmins, orchestrate early inflammatory events is poorly understood. Herein we reveal that IL-33, an alarmin sequestered in the lung epithelium, is required to limit inflammation after injury due to an unappreciated capacity to mediate Foxp3+ Treg control of local cytokines and myeloid populations. Specifically, Il33-/- mice are more susceptible to lung damage-associated morbidity and mortality that is typified by augmented levels of the proinflammatory cytokines and Ly6Chi monocytes in the bronchoalveolar lavage fluid. Local delivery of IL-33 at the time of injury is protective but requires the presence of Treg cells. IL-33 stimulates both mouse and human Tregs to secrete IL-13. Using Foxp3Cre × Il4/Il13fl/fl mice, we show that Treg expression of IL-13 is required to prevent mortality after acute lung injury by controlling local levels of G-CSF, IL-6, and MCP-1 and inhibiting accumulation of Ly6Chi monocytes. Our study identifies a regulatory mechanism involving IL-33 and Treg secretion of IL-13 in response to tissue damage that is instrumental in limiting local inflammatory responses and may shape the myeloid compartment after lung injury.

Recipient conditioning contributes to IL-33-driven Th1 alloimmune responses following rapid ST2 upregulation on donor CD4+ T cells during lymphopenia-induced proliferation

IL-33 is augmented in recipient tissues by conditioning before allogeneic stem cell transplantation (alloSCT) and is a required signal to donor T cell for GVHD. Targetable mechanisms by which IL-33 supports GVHD are yet undefined. Conditioning causes lymphopenia-induced proliferation (LIP) and releases microbial products. These products promote myeloid cell secretion of IL-12, which induces the IL-33 receptor, ST2, on T cells in vitro. We hypothesized that IL-12 induces ST2 on donor T cells during LIP promoting IL-33 augmentation of the Th1 responses leading to GVHD. To establish the role of IL-12 in T cell ST2 expression and IL-33-mediated GVHD, irradiated BALB/c mice were given B6 alloSCT and T cells. Some mice received IL-12p40 neutralizing Ab or control Ab alone, with or without concurrent IL-33 administration. To test the impact of lymphopenia on T cells, ST2−Foxp3−CD4+ T cells alone or with ST2+ Treg were transferred into B6 Rag2−/−γc−/− mice. Mice received PBS or IL-33 on days 1–8. Neutralizing IL-12 did not reduce ST2 on T cells after alloSCT nor rescue mice from IL-33-mediated acceleration of GVHD. Post-alloSCT, IL-33 worked with IL-12 to expand ST2+Tbet+ Th1 cells. Neutralizing IL-12, but not delivery of IL-33, increased Gata3+ Th2 cells after alloSCT. During LIP, CD4+Foxp3− T cells rapidly upregulated ST2 independent of IL-33, but IL-33 then favored the expansion of Th1 cells, even in the presence of ST2+ Treg. These data suggest that CD4+ T cells rapidly upregulate ST2 during alloSCT conditioning induced lymphopenia. After alloSCT, IL-33 does not support Treg or Th2 cells, but works with IL-12 to augment Th1 responses and GVHD. Blocking stimuli that induces ST2 on proliferating donor CD4+ T cells may be effective for limiting GVHD.

IL-33-Driven Innate Tissue-Protective Function of ST2+ Treg Cells

Non-lymphoid tissue-resident CD4+ Foxp3+ regulatory T (Treg) cells with the capacity to modulate non-immunological processes including organismal metabolism and tissue repair have been recently described. Notably, a large fraction of non-lymphoid tissue-resident Treg cells express ST2, the receptor for the tissue-derived cytokine and alarmin, IL-33. However, the relationship between IL-33 and ST2+ Treg cells in quiescent and pathogenic states is only starting to be understood. Using FACS and Foxp3 reporter mice, we demonstrate that ST2+ Treg cells from naïve animals are phenotypically and functionally unique relative to ST2− Treg isolated from the same location. Yet, our studies establish that peripheral and lymphoid tissue ST2+ Treg cells are highly comparable. Interestingly, ST2+ Treg exhibit a high level of aerobic glycolysis, which supports their augmented potential for cytokine secretion, especially that of IL-5, IL-10, and IL-13. IL-5 and IL-13 secretion in response to IL-33 is an innate function of ST2+ Treg cells as it does not require TCR-stimulation. Conversely, ST2+ Treg release of other cytokines, including IL-10, is antigen-restricted and amplified by IL-33. Among Treg subsets, ST2+ Treg cells are the exclusive source of Type 2 cytokines, especially IL-13, which we reveal supports Treg reparative responses. Using mice deficient in IL-33 or Treg cells, we establish that Treg cells and IL-33 are both required to protect mice from lethal lung injury. In line with these data, delivery of IL-13, but not IL-33, at the time of lung injury rescues Treg-depleted mice. This study establishes that ST2+ Treg cells possess an innate capacity for cytokine production that is crucial to tissue repair and inflammation control.

IL-33 is an unconventional Alarmin that stimulates IL-2 secretion by dendritic cells to selectively expand IL-33R/ST2+ regulatory T cells

IL-33 is a recently characterized IL-1 family member that is proposed to function as an alarmin, or endogenous signal of cellular damage, as well as act as a pleiotropic cytokine. The ability of IL-33 to potentiate both Th1 and Th2 immunity supports its role in pathogen clearance and disease immunopathology. Yet, IL-33 restrains experimental colitis and transplant rejection by expanding regulatory T cells (Treg) via an undefined mechanism. We sought to determine the influence of IL-33 on hematopoietic cells that drives Treg expansion and underlies the therapeutic benefit of IL-33 administration. In this study, we identify a feedback loop in which conventional mouse CD11c(+) dendritic cells (DC) stimulated by IL-33 secrete IL-2 to selectively expand IL-33R(ST2(+))- suppressive CD4(+)Foxp3(+) Treg. Interestingly, this occurs in the absence of classical DC maturation, and DC-derived (innate) IL-2 increases ST2 expression on both DC and interacting Treg. ST2(+) Treg represent an activated subset of Foxp3(+) cells, demonstrated to be ICOS(high)CD44(high) compared with their ST2(-) counterparts. Furthermore, although studies have shown that IL-33-exposed DC promote Th2 responses, we reveal that ST2(+) DC are required for IL-33-mediated in vitro and in vivo Treg expansion. Thus, we have uncovered a relationship between IL-33 and innate IL-2 that promotes the selective expansion of ST2(+) Treg over non-Treg. These findings identify a novel regulatory pathway driven by IL-33 in immune cells that may be harnessed for therapeutic benefit or for robust expansion of Treg in vitro and in vivo.

Cutting edge: Flt3 ligand mediates STAT3-independent expansion but STAT3-dependent activation of myeloid-derived suppressor cells

The Flt3-Flt3 ligand (Flt3L) pathway is critically involved in the differentiation and homeostasis of myeloid cells, including dendritic cells (DC); however, its role in the expansion and function of myeloid-derived suppressor cells (MDSC) has not been determined. In this article, we describe the ability of Flt3L to expand and activate murine MDSC capable of suppressing allograft rejection upon adoptive transfer. Although Flt3L expands and augments the stimulatory capacity of myeloid DC, MDSC expanded by Flt3L have increased suppressive activity. Although STAT3 is considered the central transcription factor for MDSC expansion, inhibition and genetic ablation of STAT3 did not block, but rather augmented, Flt3L-mediated MDSC expansion. MDSC suppressive function, preserved when STAT3 inhibition was removed, was reduced by genetic STAT3 deletion. Both STAT3 inhibition and deletion reduced Flt3L-mediated DC expansion, signifying that STAT3 had reciprocal effects on suppressive MDSC and immunostimulatory DC expansion. Together, these findings enhance our understanding of the immunomodulatory properties of Flt3L.