IDO1 scavenges reactive oxygen species in myeloid-derived suppressor cells to prevent graft-versus-host disease.

Tryptophan-catabolizing enzyme indoleamine 2,3-dioxygenase 1 (IDO1) also has an immunological function to suppress T cell activation in inflammatory circumstances, including graft-versus-host disease (GVHD), a fatal complication after allogeneic bone marrow transplantation (allo-BMT). Although the mononuclear cell expression of IDO1 has been associated with improved outcomes in GVHD, the underlying mechanisms remain unclear. Herein, we used IDO-deficient (Ido1-/-) BMT to understand why myeloid IDO limits the severity of GVHD. Hosts with Ido1-/- BM exhibited increased lethality, with enhanced proinflammatory and reduced regulatory T cell responses compared with wild type (WT) allo-BMT controls. Despite the comparable expression of the myeloid-derived suppressor cell (MDSC) mediators, arginase-1, inducible nitric oxide synthase, and interleukin 10, Ido1-/- Gr-1+CD11b+ cells from allo-BMT or in vitro BM culture showed compromised immune-suppressive functions and were skewed toward the Ly6ClowLy6Ghi subset, compared with the WT counterparts. Importantly, Ido1-/-Gr-1+CD11b+ cells exhibited elevated levels of reactive oxygen species (ROS) and neutrophil numbers. These characteristics were rescued by human IDO1 with intact heme-binding and catalytic activities and were recapitulated by the treatment of WT cells with the IDO1 inhibitor L1-methyl tryptophan. ROS scavenging by N-acetylcysteine reverted the Ido1-/-Gr-1+CD11b+ composition and function to an MDSC state, as well as improved the survival of GVHD hosts with Ido1-/- BM. In summary, myeloid-derived IDO1 enhances GVHD survival by regulating ROS levels and limiting the ability of Gr-1+CD11b+ MDSCs to differentiate into proinflammatory neutrophils. Our findings provide a mechanistic insight into the immune-regulatory roles of the metabolic enzyme IDO1.

Interaction of CD99 with its paralog CD99L2 positively regulates CD99L2 trafficking to cell surfaces.

Mouse CD99 and its paralog CD99-like 2 (CD99L2) are surface proteins implicated in cellular adhesion and migration. Although their distributions overlap in a wide variety of cells, their physical/functional relationship is currently unknown. In this study, we show the interaction between the two molecules and its consequence for membrane trafficking of mouse (m)CD99L2. The interaction was analyzed by bimolecular fluorescence complementation, immunoprecipitation, and fluorescence resonance energy transfer assays. When coexpressed, mCD99 formed heterodimers with mCD99L2, as well as homodimers, and the heterodimers were localized more efficiently at the plasma membrane than were the homodimers. Their interaction was cytoplasmic domain-dependent and enhanced mCD99L2 trafficking to the plasma membrane regardless of whether it was transiently overexpressed or endogenously expressed. Surface levels of endogenous mCD99L2 were markedly low on thymocytes, splenic leukocytes, and CTL lines derived from CD99-deficient mice. Importantly, the surface levels of mCD99L2 on mCD99-deficient cells recovered significantly when wild-type mCD99 was exogenously introduced, but they remained low when a cytoplasmic domain mutant of mCD99 was introduced. Our results demonstrate a novel role for mCD99 in membrane trafficking of mCD99L2, providing useful insights into controlling transendothelial migration of leukocytes.

H60: A Unique Murine Hematopoietic Cell-Restricted Minor Histocompatibility Antigen for Graft-versus-Leukemia Effect.

Allogeneic hematopoietic stem cell transplantation (allo-HSCT) is an important treatment for many types of hematological malignancies. Matching of donor and recipient for the major histocompatibility complex (MHC) improves the HSCT reconstitution, but donor-derived T cells reactive to non-MHC encoded minor histocompatibility antigens (MiHAs) can induce graft-versus-host disease (GVHD) while also being needed for graft-versus-leukemia (GVL) effects. MiHAs are allelically variant self-peptides presented conventionally on MHC molecules, but are alloantigenic in transplantation settings. Immunodominant MiHAs are most strongly associated with GVHD and GVL. There is need for mouse paradigms to understand these contradictory effects. H60 is a highly immunodominant mouse MiHA with hematopoietic cell-restricted expression. Immunodominance of H60 is tightly associated with its allelic nature (presence vs. absence of the transcripts), and the qualitative (TCR diversity) and quantitative (frequency) traits of the reactive T cells. The identity as a hematopoietic cell-restricted antigen (HRA) of H60 assists the appearance of the immunodominace in allo-HSCT circumstances, and generation of GVL effects without induction of serious GVHD after adoptive T cell transfer. Also it allows the low avidity T cells to escape thymic negative selection and exert GVL effect in the periphery, which is a previously unevaluated finding related to HRAs. In this review, we describe the molecular features and immunobiology in detail through which H60 selectively exerts its potent GVL effect. We further describe how lessons learned can be extrapolated to human allo-HCST.

Escape from thymic deletion and anti-leukemic effects of T cells specific for hematopoietic cell-restricted antigen.

Whether hematopoietic cell-restricted distribution of antigens affects the degree of thymic negative selection has not been investigated in detail. Here, we show that T cells specific for hematopoietic cell-restricted antigens (HRA) are not completely deleted in the thymus, using the mouse minor histocompatibility antigen H60, the expression of which is restricted to hematopoietic cells. As a result, low avidity T cells escape from thymic deletion. This incomplete thymic deletion occurs to the T cells developing de novo in the thymus of H60-positive recipients in H60-mismatched bone marrow transplantation (BMT). H60-specific thymic deletion escapee CD8+ T cells exhibit effector differentiation potentials in the periphery and contribute to graft-versus-leukemia effects in the recipients of H60-mismatched BMT, regressing H60+ hematological tumors. These results provide information essential for understanding thymic negative selection and developing a strategy to treat hematological tumors.

Subdominant H60 antigen-specific CD8 T-cell response precedes dominant H4 antigen-specific response during the initial phase of allogenic skin graft rejection.

In allogeneic transplantation, including the B6 anti-BALB.B settings, H60 and H4 are two representative dominant minor histocompatibility antigens that induce strong CD8 T-cell responses. With different distribution patterns, H60 expression is restricted to hematopoietic cells, whereas H4 is ubiquitously expressed. H60-specific CD8 T-cell response has been known to be dominant in most cases of B6 anti-BALB.B allo-responses, except in the case of skin transplantation. To understand the mechanism underlying the subdominance of H60 during allogeneic skin transplantation, we investigated the dynamics of the H60-specific CD8 T cells in B6 mice transplanted with allogeneic BALB.B tail skin. Unexpectedly, longitudinal bioluminescence imaging and flow cytometric analyses revealed that H60-specific CD8 T cells were not always subdominant to H4-specific cells but instead showed a brief dominance before the H4 response became predominant. H60-specific CD8 T cells could expand in the draining lymph node and migrate to the BALB.B allografts, indicating their active participation in the anti-BALB.B allo-response. Enhancing the frequencies of H60-reactive CD8 T cells prior to skin transplantation reversed the immune hierarchy between H60 and H4. Additionally, H60 became predominant when antigen presentation was limited to the direct pathway. However, when antigen presentation was restricted to the indirect pathway, the expansion of H60-specific CD8 T cells was limited, whereas H4-specific CD8 T cells expanded significantly, suggesting that the temporary immunodominance and eventual subdominance of H60 could be due to their reliance on the direct antigen presentation pathway. These results enhance our understanding of the immunodominance phenomenon following allogeneic tissue transplantation.

Memory programming in CD8(+) T-cell differentiation is intrinsic and is not determined by CD4 help.

CD8(+) T cells activated without CD4(+) T-cell help are impaired in memory expansion. To understand the underlying cellular mechanism, here we track the dynamics of helper-deficient CD8(+) T-cell response to a minor histocompatibility antigen by phenotypic and in vivo imaging analyses. Helper-deficient CD8(+) T cells show reduced burst expansion, rapid peripheral egress, delayed antigen clearance and continuous activation, and are eventually exhausted. Contrary to the general consensus that CD4 help encodes memory programmes in CD8(+) T cells and helper-deficient CD8(+) T cells are abortive, these cells can differentiate into effectors and memory precursors. Importantly, accelerating antigen clearance or simply increasing the burst effector size enables generation of memory cells by CD8(+) T cells, regardless of CD4 help. These results suggest that the memory programme is CD8(+) T-cell-intrinsic, and provide insight into the role of CD4 help in CD8(+) T-cell responses.