Mechanisms of allografted tumor rejection: the roles of T cells in allograft rejection mediated by a type of bone marrow-derived macrophage

Transplantation of Graft Anti-Host Cytotoxic T Lymphocytes Along with Allogeneic Bone Marrow Skips Macrophage-Induced Graft-Versus-Host Disease

Graft-versus-host disease (GVHD) is a physiological response of the graft to allogeneic hosts. However, the effector cells, affected organ(s), and cytokines in the GVHD remain controversially discussed, without having determined a particular cytotoxic activity of the graft against the host. After i.v. injection of C57BL/6 (H-2^b) spleen cells into irradiated BDF1 (H-2^b/d) mice, the hosts developed interferon-gamma (IFN-γ)-dependent bone marrow (BM) GVHD on days 5-17. When H-2D^dK^d transgenic H-2^b lymphoma cells were i.p. inoculated into irradiated, H-2^b splenocyte-transplanted H-2^b/d mice, the infiltration of macrophages cytotoxic against H-2D^dK^d transgenic H-2^b mouse skin epithelia (a GVHD activity) into the peritoneal cavity preceded several days the infiltration of interleukin (IL)-2-dependent cytotoxic T lymphocytes (CTLs) to achieve a graft-versus-leukemia (GVL) effect. In contrast, allogeneic BM transplanted alone into the irradiated mice did not induce GVHD for 44 days, whereas i.v. injection of graft anti-host macrophages or graft anti-host CTLs along with allogeneic BM, respectively, induced GVHD or promoted the GVL effect in the absence of GVHD. These results revealed that macrophage-induced GVHD and the CTL-mediated GVL effect were a set (Th1: IFN-γ/IL-2) response of the graft to allogeneic hosts and leukemia cells, respectively, and that graft T cell activation rather than inhibition skipped GVHD after BM transplantation.

Down-regulated expression of monocyte/macrophage major histocompatibility complex receptors in human and mouse monocytes by expression of their ligands

Mouse monocyte/macrophage major histocompatibility complex (MHC) receptor 1 (MMR1; or MMR2) specific for H-2D(d) (or H-2K(d) ) molecules is expressed on monocytes from non-H-2D(d) (or non-H-2K(d) ), but not those from H-2D(d) (or H-2K(d) ), inbred mice. The MMR1 and/or MMR2 is essential for the rejection of H-2D(d) - and/or H-2K(d) -transgenic mouse skin onto C57BL/6 (H-2D(b) K(b) ) mice. Recently, we found that human leucocyte antigen (HLA)-B44 was the sole ligand of human MMR1 using microbeads that had been conjugated with 80 types of HLA class I molecules covering 94·2% (or 99·4%) and 92·4% (or 96·2%) of HLA-A and B molecules of Native Americans (or Japanese), respectively. In the present study, we also explored the ligand specificity of human MMR2 using microbeads. Microbeads coated with HLA-A32, HLA-B13 or HLA-B62 antigens bound specifically to human embryonic kidney (HEK)293T or EL-4 cells expressing human MMR2 and to the solubilized MMR2-green fluorescent protein (GFP) fusion protein; and MMR2(+) monocytes from a volunteer bound HLA-B62 molecules with a Kd of 8·7 × 10(-9)  M, implying a three times down-regulation of MMR2 expression by the ligand expression. H-2K(d) (or H-2D(d) ) transgene into C57BL/6 mice down-regulated not only MMR2 (or MMR1) but also MMR1 (or MMR2) expression, leading to further down-regulation of MMR expression. In fact, monocytes from two (i.e. MMR1(+) /MMR2(+) and MMR1(-) /MMR2(-) ) volunteers bound seven to nine types of microbeads among 80, indicating ≤ 10 types of MMR expression on monocytes. The physiological role of constitutive MMRs on monocytes possibly towards allogeneic (e.g. fetal) cells in the blood appears to be distinct from that of inducible MMRs on macrophages toward allografts in tissue.

MHC class I recognition by monocyte-/macrophage-specific receptors

The most important transplantation antigens in the discrimination between "self" and "nonself" are encoded by genes in the major histocompatibility complex (MHC) locus (H-2 in mice). It has been assumed that T lymphocytes are the effector cells for allograft rejection, as athymic nude rodents fail to reject allografts. In 1988, we i.p. transplanted Meth A (H-2D(d)K(d)) tumor cells into C57BL/6 (H-2D(b)K(b)) mice and found macrophages to be cytotoxic against the allografts. In 1996, several groups using CD4 or CD8 knockout mice reported that non-T cells were the effector cells for the rejection of skin or organ allografts. In 1998, we ascertained that macrophages were the effector cells of skin allograft rejection. Recently, we isolated cDNA clones encoding monocyte/macrophage MHC receptors (MMRs) for H-2D(d) and H-2K(d); established H-2D(d)- and/or H-2K(d)-transgenic mice and lymphoma cells; and found, using MMR-deficient mice, that MMR and T-cell receptor were essential for the rejection of transgenic skin and lymphoma, respectively.

Infiltration of H-2d-Specific Cytotoxic Macrophage with Unique Morphology into Rejection Site of Allografted Meth A (H-2d) Tumor Cells in C57BL/6 (H-2b) Mice

It is assumed that CD8(+) cytotoxic T lymphocytes (CTLs) mediate direct lysis of allografts and that their growth, differentiation, and activation are dependent upon cytokine production by CD4(+) helper T lymphocytes. In the present study, the effector cells responsible for the rejection of i.p. allografted, CTL-resistant Meth A tumor cells from C57BL/6 mice were characterized. The cytotoxic activity was associated exclusively with peritoneal exudate cells and not with the cells in lymphoid organs or blood. On day 8, when the cytotoxic activity reached a peak, 3 types of cells (i.e., lymphocytes, granulocytes, and macrophages) infiltrated into the rejection site; and allograft-induced macrophages (AIM) were cytotoxic against the allograft. Bacterially-elicited macrophages also exhibited cytotoxic activity (approximately 1/2 of that of AIM) against Meth A cells, whereas the cytotoxic activity of AIM against these cells but not that of bacterially-elicited macrophages was completely inhibited by the addition of donor (H-2(d))-type lymphoblasts, suggesting H-2(d)-specific cytotoxicity of AIM against Meth A cells. In contrast, resident macrophages were inactive toward Meth A cells. Morphologically, the three-dimensional appearance of AIM showed them to be unique large elongated cells having radiating peripheral filopodia and long cord-like extensions arising from their cytoplasmic surfaces. The ultrastructural examination of AIM revealed free ribosomes in their cytoplasm, which was often deformed by numerous large digestive vacuoles. These results indicate that AIM are the H-2(d)-specific effector cells for allografted Meth A cells and are a more fully activated macrophage with unique morphological features.

A Novel Receptor on Allograft (H-2d)-Induced Macrophage (H-2b) toward an Allogeneic Major Histocompatibility Complex Class I Molecule, H-2Dd, in Mice

The generation of knockout mice demonstrated that noncytotoxic CD4(+), but not cytotoxic CD8(+), T cells were essential for the rejection of skin or organ allografts. Earlier we reported that allograftinduced macrophages (AIM) in mice lysed allografts with H-2 haplotype specificity, implying screening of grafts by AIM. Here, we isolated a cDNA clone encoding a novel receptor on AIM (H-2D(b)) for an allogeneic major histocompatibility complex (MHC) class I molecule, H-2D(d), by using H-2D(d) tetramer and a monoclonal antibody (mAb; R15) specific for AIM. The cDNA (1,181-bp) encoded a 342-amino acid polypeptide with a calculated molecular mass of 45 kDa and was found to be expressed on AIM, but not on resident macrophages or other cells, infiltrating into the rejection site. HEK293T cells transfected with this cDNA reacted with R15 mAb and H-2D(d), but not H-2L(d), H-2K(d), H-2D(b), H-2K(b), H-2D(k), or H-2K(k), molecules; and the H-2D(d) binding was suppressed by the addition of R15 or anti-H-2D(d) mAb. AIM yielded a specific saturation isotherm in the presence of increasing concentrations of H-2D(d), but not H-2D(b) or H-2D(k), molecules. The dissociation constant of AIM toward H-2D(d) tetramers was 1.9 x 10(-9) M ; and the binding was completely inhibited by the addition of R15 or anti-H-2D(d) mAb. These results reveal that a novel receptor for an allogeneic H-2D(d) molecule was induced on effector macrophages responsible for allograft (H-2(d)) rejection in H-2(b) mice.

Differential Susceptibility of Cells Expressing Allogeneic MHC or Viral Antigen to Killing by Antigen-Specific CTL

CD8(+) cytotoxic T lymphocytes (CTLs) generated by immunization with allogeneic cells or viral infection are able to lyse allogeneic or virally infected in vitro cells (e.g., lymphoma and mastocytoma). In contrast, it is reported that CD8(+) T cells are not essential for allograft rejection (e.g., heart and skin), and that clearance of influenza or the Sendai virus from virus-infected respiratory epithelium is normal or only slightly delayed after a primary viral challenge of CD8-knockout mice. To address this controversy, we generated H-2(d)-specific CD8(+) CTLs by a mixed lymphocyte culture and examined the susceptibility of a panel of H-2(d) cells to CTL lysis. KLN205 squamous cell carcinoma, Meth A fibrosarcoma, and BALB/c skin components were found to be resistant to CTL-mediated lysis. This resistance did not appear to be related to a reduced expression of MHC class I molecules, and all these cells could block the recognition of H-2(d) targets by CTLs in cold target inhibition assays. We extended our observation by persistently infecting the same panel of cell lines with defective-interfering Sendai virus particles. The Meth A and KLN205 lines infected with a variant Sendai virus were resistant to lysis by Sendai virus-specific CTLs. The Sendai virus-infected Meth A and KLN205 lines were able to block the lysis of Sendai virus-infected targets by CTLs in cold target inhibition assays. Taken together, these results suggest that not all in vivo tissues may be sensitive to CTL lysis.

Interleukin-4 enhances trafficking and functional activities of GM-CSF-stimulated mouse myeloid-derived dendritic cells at late differentiation stage

Mouse bone marrow-derived dendritic cells (BMDCs) are being employed as an important model for translational research into the development of DC-based therapeutics. For such use, the localization and specialized mobility of injected BMDCs within specific immune tissues are known to define their immunity and usefulness in vivo. In this study, we demonstrate that IL-4, a key driving factor for in vitro propagation and differentiation of BMDCs, when added during a late culture stage can enhance the in vivo trafficking activity of granulocyte-macrophage colony-stimulating factor (GM-CSF)-induced BMDCs. It suggests that the temporal control of IL-4 stimulation during the in vitro generation of DCs drastically affects the DC trafficking efficiency in vivo. With this modification of IL-4 stimulation, we also show that much less cytokine was needed to generate BMDCs with high purity and yield that secrete a high level of cytokines and possess a good capacity to induce proliferation of allogeneic CD4+ T cells, as compared to the conventional method that uses a continuous supplement of GM-CSF and IL-4 throughout cultivation. These results provide us with an important know-how for differentiation of BMDCs from myeloid stem cells, and for use of other immune cells in related medical or stem cell applications.

The role of macrophages in xenograft rejection

Safe and effective xenotransplantation would provide a valuable answer to many of the limitations of allogenic transplantation. Such limitations include scarcity of organ supply and morbidity to donors in cases of living-related donor transplantation. The main hurdle to the efficacious application of xenotransplantation in clinical medicine is the fierce host immune response to xenografts. This immune response is embodied in 3 different types of xenograft rejection. Both hyperacute rejection and delayed xenograft rejection are mediated by natural antibodies and are concerned primarily with whole organ rejection. Cellular xenograft rejection (CXR), on the other hand, is concerned with both whole organ and CXR and is mediated by innate immunity rather than natural antibodies. Macrophages, which are cells of the innate immune system, play a role in all 3 types of xenograft rejection (not just CXR). They impart their effects both directly and through T-cell activation.

Acute Rejection of Allografted CTL-Susceptible Leukemia Cells from Perforin/Fas Ligand Double-Deficient Mice

The generation of knockout mice demonstrated that CD4(+), but not CD8(+), T cells were essential for the rejection of allografted skin or heart, presumably because these targets were CTL resistant. In the case of CTL-susceptible targets (e.g., P815 mastocytoma cells and EL-4 or RLmale1 T lymphoma cells), however, it is assumed that the CTL is the effector cell responsible for allograft rejection and that perforin and Fas ligand (FasL) pathways are the killing mechanisms. In the present study, we examined the role of these cytotoxic molecules in the rejection of i.p. allografted CTL-susceptible leukemia cells. Unexpectedly, the allografted leukemia cells were acutely rejected from gld (a mutation of FasL), perforin(-/-), or double-deficient mice. The peritoneal exudate cells from gld or normal mice showed T cell-, TCRalphabeta-, and perforin-dependent cytotoxic activity against the allograft, whereas the exudate cells from perforin(-/-) mice exhibited almost full cytotoxic activity in the presence of Fas-Fc. Furthermore, the infiltrates from double-deficient mice showed a high cytotoxic activity against the allografted cells even in the presence of anti-TCRalphabeta Ab or in the absence of T cells. The cytotoxic cells appeared to be macrophages, because they were Mac-1(+) mononuclear cells with a kidney- or horseshoe-shaped nucleus and because the cytotoxic activity was completely suppressed by the addition of N(G)-monomethyl-l-arginine, an inhibitor of inducible NO synthase. These results indicate that macrophages are ready and available to kill CTL-susceptible allografts when CTLs lack both perforin and FasL molecules.

Macrophage MHC receptor 2: A novel receptor on allograft (H-2DdKd)-induced macrophage (H-2DbKb) recognizing an MHC class I molecule, H-2Kd, in mice

We previously reported that a population of allograft (H-2D(d)K(d))-induced macrophages (AIM) in C57BL/6 (H-2D(b)K(b)) mice exhibited major histocompatibility complex (MHC) haplotype (H-2D(d)) specific killing of the allograft (e.g., BALB/c skin and Meth A cells; H-2D(d)K(d)) in a macrophage MHC receptor (MMR)-dependent manner. In the present study, we isolated a cDNA clone encoding a novel receptor (MMR2) on AIM recognizing another MHC class I molecule, H-2K(d), by the expression cloning method using H-2K(d) tetramer and a monoclonal antibody (mAb; R12) specific for AIM. The cDNA (2359-bp) encoded a 677-amino acid polypeptide of a calculated molecular mass of 87 kDa and was found to be expressed exclusively on AIM among cells infiltrating into allografts on days 0-9 after transplantation. Confocal microscopy showed that HEK293T cells transfected with this cDNA were reactive toward the H-2K(d) molecule but not toward other MHC class I molecules such as H-2D(d), H-2D(b), H-2D(k), H-2K(b), H-2K(k), and H-2L(d) molecules. The binding of the H-2K(d) molecule to the transfectants was inhibited by the addition of R12 or anti-H-2K(d), but not by R15 (a mAb specific for H-2D(d) receptor) or anti-H-2D(d), mAb. Flow cytometric analysis revealed specific binding of H-2K(d) molecules to AIM (K(d)=2.7x10(-9) M); and the binding was completely suppressed by the addition of R12 mAb. These results demonstrate that a novel receptor (MMR2) for H-2K(d) molecules was induced on effector macrophages responsible for allograft (H-2D(d)K(d)) rejection by H-2D(b)K(b) mice.

IFN-γ: A Cytokine Essential for Rejection of CTL-Resistant, Virus-Infected Cells

We recently demonstrated differential susceptibility of cells expressing viral antigen to killing by antigen-specific cytotoxic T lymphocytes (CTLs). In addition, interferon-gamma (IFN-gamma) has been implicated in the clearance of some viruses from tissues. We explored the role of IFN-gamma in the cytotoxicity of Sendai virus-specific CTLs against virus-infected RL(male symbol)1 (T cell leukemia) or Meth A (fibrosarcoma) cells, as well as the growth of subcutaneously (s.c.) transplanted, virus-infected cells in IFN-gamma(+/+) or IFN-gamma(/) mice of the syngeneic strain (BALB/c). Sendai virus-specific CTLs were cytotoxic against virus-infected RL(male symbol)1 cells, and s.c. transplanted, virus-infected RL(male symbol)1 cells were acutely rejected from IFN-gamma(+/+) or IFN-gamma(/) mice. In contrast, the CTLs were inactive toward virus-infected Meth A cells, but s.c. transplanted, virus-infected Meth A cells were acutely rejected from IFN-gamma(+/+) but not IFN-gamma(/) mice. The s.c. growth of virus-infected Meth A cells in the mutant mice was markedly inhibited by s.c. injections of IFN-gamma, and the rejection from IFN-gamma(+/+) mice was delayed after specific elimination of macrophages by intravenous (i.v.) injections of dichloromethylene diphosphonatecontaining liposomes. These results suggest an essential role of IFN-gamma and involvement of macrophage in the rejection of CTL-resistant, virus-infected cells.

Induction of hair regrowth in the alopecia site of IFN-gamma knockout mice by allografting and IFN-gamma injection into the transplantation site

We previously demonstrated that around 6 weeks of age most of the interferon-gamma (IFN-gamma)(-/-) but none of the IFN-gamma(+/+) C57BL/6 mice began to lose hair in their dorsal or occipital areas or both and that a single s.c. injection of IFN-gamma into IFN-gamma(-/-) mice at 3 but not at 8 weeks of age (or later) could protect all the mice from alopecia. Here, we report hair regrowth in the alopecia site of IFN-gamma(-/-) mice at 8 weeks of age (or later) by the combination of IFN-gamma and allografting. Skin or tumor allografting and IFN-gamma injections into the transplantation site induced hair regrowth in the alopecia site of IFN-gamma(-/-) mice at 8-66 weeks of age, whereas IFN-gamma injections into the hairless site or allografting alone was ineffective in causing the hair regrowth. Histologic findings showed that the hair cycle in the region of alopecia of IFN-gamma(-/-) mice was blocked at the anagen stage and that in the IFN-gamma(-/-) mice treated with IFN-gamma and allografting, the cycle was at the telogen stage. The therapeutic effects were maintained for >1 year.

Innate and adaptive immune responses to nonvascular xenografts: evidence that macrophages are direct effectors of xenograft rejection

Nonvascularized xenograft rejection is T cell mediated, but is dependent on initial macrophage (Mphi) infiltration. We developed an i.p. transplant model to define the roles of Mphi and T cells in xenograft rejection. Nonobese diabetic or BALB/c mice were injected i.p. with xenogeneic, allogeneic, or syngeneic cells, and the responding cells in subsequent lavages were assessed by flow cytometry and adoptive transfer. Neutrophils and monocytes/elicited Mphi were rapidly recruited in response to xenogeneic pig (PK15 or spleen) cells and, to a significantly lesser extent, allogeneic cells. These innate responses preceded T cell infiltration and occurred in their absence in SCID mice. Syngeneic cells induced negligible neutrophil or Mphi responses. Neutrophils and Mphi induced by xenogeneic cells in SCID mice stimulated T cell recruitment after transfer to immunocompetent mice. T cells in turn were required for Mphi activation and xenogeneic cell rejection. Thus, Mphi harvested from immunocompetent but not SCID mice injected with xenogeneic cells expressed activation markers and rejected xenogeneic cells when transferred into SCID mice. These findings demonstrate the interdependent roles of Mphi and T cells in xenograft rejection. The requirement for Mphi reflects their ability to mount a rapid, local innate response that stimulates T cell recruitment and, having received T cell help, to act as direct effectors of rejection.

Leukocyte integrin-dependent and antibody-independent cytotoxicity of macrophage against allografts

Macrophages (Mphis), but not T cells, infiltrating into the rejection site of either i.p. allografted Meth A (H-2d) fibrosarcoma cells in C57BL/6 (B6) (H-2b) mice or BALB/c (H-2d) skin onto B6 mice are cytotoxic against allografts with H-2d specificity. To determine the mechanisms of specific killing of allografts by allograft-induced Mphi (AIM), we raised approximately 5,000 rat monoclonal antibodies (mAbs) against AIM and selected three of them (R1-73, R2-40 and R1-34), each of which inhibited cytotoxic activity against allografts in a dose-dependent manner. The antigens recognized by R1-73, R2-40 and R1-34 mAbs were defined by immunoprecipitation and Western blot analyses as CD11a, CD18 and CD11b, respectively; and the allografts expressed CD54, a ligand of CD11a or CD11b, suggesting leukocyte integrin-dependent killing. Although Ab-dependent cellular cytotoxicity has been recognized as a mechanism of specific killing by Mphis, the infiltration of AIM into the rejection site of allografts far (approximately 6 days) preceded the appearance of serum IgG Ab specific for the allograft. AIM exhibiting full cytotoxic activity against allografts was also induced in the transplantation site of Fcgamma receptor knockout [(B6x129) F1] mice as well as B10.D2 (H-2 compatible with allograft) and B6-xid (X-linked immunodeficiency with B cell-specific defect) strains of mice. In the latter two strains of mice, the levels of serum IgG Ab to the allograft were negligible. Moreover, the cytotoxic activity of AIM against allografts was not affected by pretreatment of the cells with anti-mouse IgG serum, suggesting Ab-independent cytotoxicity.

IFN-γ- and Cell-to-Cell Contact-Dependent Cytotoxicity of Allograft-Induced Macrophages Against Syngeneic Tumor Cells and Cell Lines: An Application of Allografting to Cancer Treatment

In allogeneic tumor or skin transplantation, the rejection process that destroys the allogeneic cells leaves syngeneic cells intact by discrimination between self and nonself. Here, we examined whether the cells infiltrating into the allografts could be cytotoxic against syngeneic immortal cells in vitro and in vivo. The leukocytes (i.e., macrophages (Mφ; 55–65% of bulk infiltrates), granulocytes (20–25%), and lymphocytes (15–20%)) infiltrating into allografts, but not into autografts, in C57BL/6 mice were cytotoxic against syngeneic tumor cells and cell lines, whereas the cytotoxic activity was hardly induced in allografted, IFN-γ−/− C57BL/6 mice. Among the leukocytes, Mφ were the major population of cytotoxic cells; and the cytotoxic activity appeared to be cell-to-cell contact dependent. When syngeneic tumor cells were s.c. injected into normal C57BL/6 mice simultaneously with the Mφ-rich population or allogeneic, but not syngeneic, fibroblastic cells, tumor growth was suppressed in a cell number-dependent manner, and tumor cells were rejected either with a Mφ:tumor ratio of about 30 or with an allograft:tumor ratio of ∼200. In the case of IFN-γ−/− C57BL/6 mice, however, the s.c. injection of the allograft simultaneously with tumor cells had no effect on the tumor growth. These results suggest that allograft or allograft-induced Mφ may be applicable for use in cancer treatment and that IFN-γ induction by the allograft may be crucial for the treatment.

The Role of T Cells in Allografted Tumor Rejection: IFN-γ Released from T Cells Is Essential for Induction of Effector Macrophages in the Rejection Site

Allografted Meth A tumor rejection is T cell dependent, but T cells are inactive toward the allograft; rather, the main effector cells are allograft-induced macrophages (AIM) with MHC haplotype specificity. Here, we examined the role of T cells in the induction of AIM in the rejection site. On day 4.5 after i.p. transplantation of Meth A fibrosarcoma cells to C57BL/6 (B6) mice, we obtained a kind of precursor of AIM (pro-AIM) from the transplantation site by an enrichment technique involving adherence to serum-coated dishes. The noncytotoxic pro-AIM-rich population put into a diffusion chamber became cytotoxic against Meth A cells after 2 days in the peritoneal cavity of an untreated B6 mouse. Similar activation of the chambered B6 pro-AIM-rich population occurred in IFN-γ −/− B6 mice, whereas there was no activation when chambers containing an IFN-γ −/− mouse-derived pro-AIM-rich population were placed in normal or IFN-γ −/− mice, suggesting that IFN-γ is involved in the activation. RT-PCR experiments demonstrated that among bulk infiltrates T cells were the major producer of IFN-γ; and most of the cells in a T cell-eliminated pro-AIM population in a diffusion chamber kept for 2 days in a B6 mouse did not become AIM. Furthermore, IFN-γ −/− B6 mice could not reject allografted Meth A tumor cells, whereas the grafts were rejected by i.p. injections of IFN-γ into the mutant mice. These results indicate that IFN-γ released from allograft-induced T cells is essential for both the activation of a kind of pro-AIM to AIM in the transplantation site and the rejection of an allografted tumor.