Critical role of NKT cells in posttransplant alloantibody production.

We previously reported that posttransplant alloantibody production in CD8-deficient hosts is IL-4+ CD4+ T cell-dependent and IgG1 isotype-dominant. The current studies investigated the hypothesis that IL-4-producing natural killer T cells (NKT cells) contribute to maximal alloantibody production. To investigate this, alloantibody levels were examined in CD8-deficient WT, CD1d KO and Jα18 KO transplant recipients. We found that the magnitude of IgG1 alloantibody production was critically dependent on the presence of type I NKT cells, which are activated by day 1 posttransplant. Unexpectedly, type I NKT cell contribution to enhanced IgG1 alloantibody levels was interferon-γ-dependent and IL-4-independent. Cognate interactions between type I NKT and B cells alone do not stimulate alloantibody production. Instead, NKT cells appear to enhance maturation of IL-4+ CD4+ T cells. To our knowledge, this is the first report to substantiate a critical role for type I NKT cells in enhancing in vivo antibody production in response to endogenous antigenic stimuli.

Generation of cellular immunity to lymphocytic choriomeningitis virus is independent of CD1d1 expression.

CD1 molecules are cell surface glycoproteins, structurally similar to major histocompatibility complex (MHC) class I molecules. The murine CD1d1 molecule has been shown to be essential for the positive selection of a unique subpopulation of T cells [the natural killer (NK) T cells], as CD1d1-deficient mice lack NK T cells. These cells have recently been suggested to play an important role in the induction of innate immunity (i.e. NK cells) and the regulation of immune homeostasis. As such, it was asked whether NK T cells were necessary for the generation of cellular immunity to an acute virus infection. In these studies, the Armstrong strain of lymphocytic choriomeningitis virus (LCMV), a classic inducer of NK cells, and its pathogenic variant clone 13 were used. When NK-cell activity was assessed on day 3 post-LCMV infection, surprisingly, it was found that CD1d1-deficient mice could generate NK-cell activity at wild-type levels. Likewise, LCMV-specific cytotoxic T-lymphocyte (CTL) activity in CD1d1-deficient mice was indistinguishable from that generated in wild-type mice. Additionally, viral titres in the spleen (LCMV Armstrong) and blood (LCMV clone 13) of infected CD1d1-deficient mice were at comparable levels to those found in wild-type mice, as were virus infection-induced increases in cell surface H-2Kb in the spleen. Therefore, these results suggest that the LCMV-induced generation of NK-cell and virus-specific CTL activity, as well as viral clearance, are independent of CD1d1 expression.

Selective loss of natural killer T cells by apoptosis following infection with lymphocytic choriomeningitis virus.

Natural killer T (NKT) cells, a unique subpopulation of T cells, coexpress markers also present on NK cells and recognize the major histocompatibility complex class I-like CD1d1 molecule. We studied the effect of an acute virus infection on NKT cells. Mice were infected with the nonhepatotropic Armstrong strain of lymphocytic choriomeningitis virus (LCMV), and at various times postinfection, mononuclear cells from the liver, peritoneum, and spleen were isolated. It was found that within 2 to 3 days, there was a selective loss of NKT cells from the liver with an apparent rapid recovery within 8 to 14 days. There was no increase in peritoneal or splenic NKT cells, indicating that NKT cells did not traffic to these tissues. This loss of NKT cells was independent of gamma interferon (IFN-gamma) and interleukin 12 (IL-12) production, but did occur in mice treated with poly(I-C), a classical inducer of IFN-alpha/beta. The reduction in NKT cells was CD28 and fas/fasL independent and occurred via apoptosis. It was not observed in LCMV-infected DNA fragmentation factor 45-deficient mice, and an increase in active caspase 3-specific staining was found in liver NKT cells from LCMV-infected and poly(I-C)-treated mice compared to uninfected wild-type mice. Interestingly, it was also found that liver NKT cells from LCMV-infected mice were themselves infected. These results suggest that the loss of NKT cells following an acute LCMV infection could be due to the induction of IFN-alpha/beta resulting in NKT-cell apoptosis and is important for the host's immune response to LCMV.

Heterosubtypic immunity to influenza A virus in mice lacking IgA, all Ig, NKT cells, or gamma delta T cells.

The mechanisms of broad cross-protection to influenza viruses of different subtypes, termed heterosubtypic immunity, remain incompletely understood. We used knockout mouse strains to examine the potential for heterosubtypic immunity in mice lacking IgA, all Ig and B cells, NKT cells (CD1 knockout mice), or gamma(delta) T cells. Mice were immunized with live influenza A virus and compared with controls immunized with unrelated influenza B virus. IgA(-/-) mice survived full respiratory tract challenge with heterosubtypic virus that was lethal to controls. IgA(-/-) mice also cleared virus from the nasopharynx and lungs following heterosubtypic challenge limited to the upper respiratory tract, where IgA has been shown to play an important role. Ig(-/-) mice controlled the replication of heterosubtypic challenge virus in the lungs. Acute depletion of CD4+ or CD8+ T cell subsets abrogated this clearance of virus, thus indicating that both CD4+ and CD8+ T cells are required for protection in the absence of Ig. These results in Ig(-/-) mice indicate that CD4+ T cells can function by mechanisms other than providing help to B cells for the generation of Abs. Like wild-type mice, CD1(-/-) mice and gamma(delta) (-/-) mice survived lethal heterosubtypic challenge. Acute depletion of CD4+ and CD8+ cells abrogated heterosubtypic protection in gamma(delta) (-/-) mice, but not B6 controls, suggesting a contribution of gamma(delta) T cells. Our results demonstrate that the Ab and cellular subsets deficient in these knockout mice are not required for heterosubtypic protection, but each may play a role in a multifaceted response that as a whole is more effective than any of its parts.

Multiple antigen-specific processing pathways for activating naive CD8+ T cells in vivo.

Current knowledge of the processing of viral Ags into MHC class I-associated ligands is based almost completely on in vitro studies using nonprofessional APCs (pAPCs). This is two steps removed from real immune responses to pathogens and vaccines, in which pAPCs activate naive CD8(+) T cells in vivo. Rational vaccine design requires answers to numerous questions surrounding the function of pAPCs in vivo, including their abilities to process and present peptides derived from endogenous and exogenous viral Ags. In the present study, we characterize the in vivo dependence of Ag presentation on the expression of TAP by testing the immunogenicity of model Ags synthesized by recombinant vaccinia viruses in TAP1(-/-) mice. We show that the efficiency of TAP-independent presentation in vitro correlates with TAP-independent activation of naive T cells in vivo and provide the first in vivo evidence for proteolytic processing of antigenic peptides in the secretory pathway. There was, however, a clear exception to this correlation; although the presentation of the minimal SIINFEKL determinant from chicken egg OVA in vitro was strictly TAP dependent, it was presented in a TAP-independent manner in vivo. In vivo presentation of the same peptide from a fusion protein retained its TAP dependence. These results show that determinant-specific processing pathways exist in vivo for the generation of antiviral T cell responses. We present additional findings that point to cross-priming as the likely mechanism for these protein-specific differences.

Impaired assembly yet normal trafficking of MHC class I molecules in Tapasin mutant mice.

Loading of peptides onto major histocompatibility complex class I molecules involves a multifactorial complex that includes tapasin (TPN), a membrane protein that tethers empty class I glycoproteins to the transporter associated with antigen processing. To evaluate the in vivo role of TPN, we have generated Tpn mutant mice. In these animals, most class I molecules exit the endoplasmic reticulum (ER) in the absence of stably bound peptides. Consequently, mutant animals have defects in class I cell surface expression, antigen presentation, CD8+ T cell development, and immune responses. These findings reveal a critical role of TPN for ER retention of empty class I molecules. Tpn mutant animals should prove useful for studies on alternative antigen-processing pathways that involve post-ER peptide loading.

Natural ligand of mouse CD1d1: cellular glycosylphosphatidylinositol.

Mouse CD1d1, a member of the CD1 family of evolutionarily conserved major histocompatibility antigen-like molecules, controls the differentiation and function of a T lymphocyte subset, NK1+ natural T cells, proposed to regulate immune responses. The CD1d1 crystal structure revealed a large hydrophobic binding site occupied by a ligand of unknown chemical nature. Mass spectrometry and metabolic radiolabeling were used to identify cellular glycosylphosphatidylinositol as a major natural ligand of CD1d1. CD1d1 bound glycosylphosphatidylinositol through its phosphatidylinositol aspect with high affinity. Glycosylphosphatidylinositol or another glycolipid could be a candidate natural ligand for CD1d1-restricted T cells.

TAP-independent, beta 2-microglobulin-dependent surface expression of functional mouse CD1.1.

CD1 molecules consist of beta 2-microglobulin (beta 2m) noncovalently complexed to a non-major histocompatibility complex (MHC)-encoded monomorphic integral membrane protein homologous to MHC class I alpha chains. Little is known about the requirements for cell surface expression and T cell recognition of CD1. We inserted the mouse CD1.1 gene into vaccinia virus to create a recombinant virus expressing CD1.1 under the control of a viral promoter. Using this recombinant virus to infect normal or mutant cell lines, we found that the expression of molecules reactive with the CD1.1-specific monoclonal antibody 3C11 requires the expression of beta 2m but was not affected by the absence of the MHC-encoded peptide transporter (TAP). Consistent with these results, IL-2 production by the mCD1.1-specific T cell hybridoma DN32.D3 was induced by thymocytes from normal mice or mice with a homozygous deletion of the TAP1 gene, but not by thymocytes from mice with a homozygous deletion of the beta 2m gene. These results indicate that expression of functional mCD1.1 occurs in a beta 2m-dependent, TAP-independent manner.

CD1 recognition by mouse NK1+ T lymphocytes.

Rare major histocompatibility complex (MHC) class I-like CD1-specific T cells have been isolated from human blood, but it has not been determined whether these clones are part of a defined subset of CD1-specific T cells selected during T cell development, or whether their recognition of CD1 is a fortuitous cross-reaction. In mice, an entire subset of alpha beta thymocytes with a unique phenotype was found to be CD1-specific. This particular subset, and its human counterpart, provide evidence that CD1 has a general role in selecting and interacting with specialized alpha beta T cells.

NK cell response to viral infections in beta 2-microglobulin-deficient mice.

Because class I MHC Ags have been implicated as modulators of target cell sensitivity to NK cell-mediated lysis, the regulation of virus infections and the fate of NK cells and their natural targets was examined in beta 2-microglobulin-deficient mice, which have defective class I MHC expression. Infections with either the NK cell-sensitive murine cytomegalovirus (MCMV) or the NK cell-resistant lymphocytic choriomeningitis virus (LCMV) significantly augmented NK cell activity in either C57BL/6 (+/+) or beta 2-microglobulin knockout (-/-) mice. Depletion of NK cells in vivo with antiserum to asialo-GM1 markedly enhanced the synthesis of MCMV but had no effect on the synthesis of LCMV in either strain of mouse. Analysis of naturally NK cell-sensitive thymocyte targets from these virus-infected -/- mice revealed no cell surface expression of class I MHC detectable by conformation-dependent or -independent Abs, but the virus infections enhanced class I expression on thymocytes from +/+ mice. The sensitivity of +/+ thymocytes to NK cell-mediated lysis was markedly reduced after in vivo poly inosinic:cytidylic and treatment or viral infection; in contrast, the sensitivity of the -/- thymocytes was significantly less affected by poly inosinic:cytidylic acid treatment or viral infection. These data indicate that the normal expression of class I MHC Ags on NK cells or their targets is not required for the antiviral functions of NK cells against a NK-sensitive virus (MCMV) nor do they protect a NK-resistant virus (LCMV) from the antiviral activity of NK cells.

Effects of virally expressed interleukin-10 on vaccinia virus infection in mice.

To investigate the in vivo role of interleukin-10 (IL-10) in viral infection, we compared infections with a recombinant vaccinia virus (VV) expressing IL-10 (VV-IL10) under control of the VV P7.5 promoter and a control virus (VV-beta gal) in normal and severe combined immunodeficient mice. In normal mice, VV-IL10 infection resulted in less natural killer cell activity at 3 days postinfection and less VV-specific cytotoxic T-cell activity at 6 or 7 days postinfection than VV-beta gal infection. However, the use of dermal scarification or intraperitoneal, intranasal, or intracerebral inoculation into immunocompetent mice resulted in no difference between VV-IL10 and VV-beta gal in visible lesions, mortality, protective immunity to a 100-fold lethal VV challenge, or VV-specific antibody response. In the immunodeficient mice, VV-IL10 infection resulted in greater natural killer cell activity and lower virus replication than VV-beta gal infection. These in vivo effects were subtler and more complex than had been anticipated. From the VV-IL10 murine model, the Epstein-Barr virus-encoded homolog of human IL-10, BCRF1, may provide a selective advantage by blunting the early human natural killer cell and cytotoxic T-cell responses so that Epstein-Barr virus can establish a well-contained latent infection in B lymphocytes.

The monoclonal antibody CZ-1 identifies a mouse CD45-associated epitope expressed on interleukin-2-responsive cells.

We have previously described a monoclonal antibody (mAb), CZ-1, which reacts with an epitope expressed on most peripheral basophils, natural killer cells, B cells, and CD8+ T cells, but not with most thymocytes or peripheral CD4+ T cells. Here we show that mAb CZ-1 defines a sialic acid-dependent epitope associated with a subpopulation of CD45 molecules. This conclusion is based on the ability to block binding of mAb CZ-1 by sialic acid, neuramin-lactose, neuraminidase, and mAb to CD45RB, and by expression of the epitope on transfected psi 2 cells expressing exon B of CD45. The results suggest that the CZ-1 epitope is a post-translational modification expressed on a subpopulation of the CD45 molecules also expressing the B exon. Expression of the CZ-1 epitope was required for freshly isolated lymphocytes to respond to interleukin-2 (IL-2). Depletion of CZ-1+ cells by C' or by cell sorting of thymocytes or splenocytes eliminated the IL-2 responsive cells. The subpopulations of thymocytes and CD4+ splenocytes responding to IL-2 were exclusively within the small CZ-1+ subpopulation. mAb CZ-1 was also used to subdivide CD45+ and CD45RB+ splenocytes into IL-2-responsive and -nonresponsive subpopulations. The CZ-1 epitope was also expressed on virtually all lymphokine-activated killer cell precursors. These data, thus, indicate that cells responsive to IL-2 express this sialated modification of CD45.

Window of vulnerability of vaccinia virus-infected cells to natural killer (NK) cell-mediated cytolysis correlates with enhanced NK cell triggering and is concomitant with a decrease in H-2 class I antigen expression.

A time course study was performed in order to determine if vaccinia virus (VV)-infected targets were more susceptible to murine natural killer (NK) cell-mediated lysis during a discrete period of time postinfection. Activated NK cells were used in short-term (e.g. 4 h) assays in order to avoid a further in vitro activation of the NK cells by interferon (IFN) and to test the innate susceptibility of target cells to lysis. The sensitivity of VV-infected L929 cells to lysis by NK cells increased as the infection progressed, reached a peak at approximately 24 h postinfection, and subsequently declined to levels lower than that of uninfected cells. This window of vulnerability was not due to an increase in the number of effector/target cell conjugates, which continually decreased as the VV infection progressed. Triggering of NK cells was measured by the influx of 45Ca2+. Target cells treated with IFN induced less 45Ca2+ uptake, whereas cycloheximide treatment of targets caused a greater influx of 45Ca2+ into the effector cells. When L929 cells were infected with VV for various time intervals and used in the triggering assays, an enhanced triggering of the effectors corresponding to the time of enhanced susceptibility of the target cells to lysis was detected. Quantitative decreases in H-2Kk and Dk class I antigens were observed following VV infection of target cells as measured by FACS analysis using alloantibodies. Qualitative changes in H-2 class I antigens were also observed, as detected by a loss in VV-infected target cell susceptibility to lysis by allospecific cytotoxic T lymphocytes (CTL) at a time when they were highly sensitive to killing by NK cells and VV-specific CTL. These results show that virus-infected targets may become innately more sensitive to lysis by NK cells at discrete time points after infection and that the susceptibility to lysis correlates with enhanced triggering of NK cells and reduced H-2 class I antigen expression.

Biological activities and antitumor mechanism of an immunopotentiating organogermanium compound, Ge-132 (review).

The biological activities and antitumor mechanism of an immunopotentiator, Ge-132, is reviewed herein. Ge-132 exhibited antitumor activity against certain syngeneic and allogeneic experimental tumors. It was shown that T-cells and macrophages were involved when tumor-bearing mice were protected by the compound. This protective effect could be transferred to tumor-bearing mice, not treated with the compound, by a macrophage fraction and serum specimens obtained from Ge-132-treated mice. Interferon gamma (IFN gamma) was detected in the circulation of Ge-132-treated mice and when sera obtained from Ge-132-treated mice were treated with anti-IFN gamma antiserum in vitro, the antitumor activity was abolished. On the other hand, in mice treated with anti-IFN gamma antiserum, Ge-132 did not induce serum IFN and failed to protect against death due to ascites tumor progression. The in vivo administration of monoclonal anti-Thy 1.2 antibody prevented the expression of the antitumor activity of Ge-132. However, serum specimens obtained from Ge-132-treated mice effectively inhibited the tumor growth of T-cell-depleted mice bearing ascites tumors. Since it has been reported that T-lymphocytes produce IFN gamma, this suggested that Ge-132 may first stimulate T-cells to produce IFN gamma in the expression of the observed antitumor efficacy. In addition, sera obtained from Ge-132-treated mice did not show any antitumor activity in mice depleted of macrophage functions. Additionally, passive transfer of macrophages from mice treated with these serum specimens to tumor-bearing mice also resulted in the inhibition of tumor growth. Pretreatment of these serum specimens with anti-IFN gamma antiserum effectively prevented the generation of cytotoxic macrophages. Also, tumor-bearing mice treated exogenously with this antiserum did not differ significantly in survival as compared to controls, despite the administration of Ge-132. Furthermore, the antitumor activity of Ge-132 was detected in NK cell-depleted mice. Therefore, the antitumor mechanism of Ge-132 in the murine ascites tumor system may be expressed as follows: (a) Ge-132 stimulates T-cells to induce IFN gamma when mice are treated orally with the compound, (b) IFN gamma activates macrophages to become cytotoxic, and (c) the cytotoxic macrophages eliminate tumor cells.

Importance of Lyt 1+ T-cells in the antitumor activity of an immunomodulator, SSM, extracted from human-type Tubercle bacilli.

An arabinomannan lipid extracted from Mycobacterium tuberculosis strain Aoyama B (SSM) is an immunopotentiating agent with interferon-inducing and antitumor activities. In the present study, the possible role(s) of various immunocompetent cells on the antitumor effect of SSM was investigated in mice bearing syngeneic (RL male 1 leukemia) and allogeneic (Ehrlich carcinoma) ascites tumors. When Thy 1+ T-cells were depleted from tumor-bearing mice by the administration of monoclonal anti-Thy 1.2 antibody, the protective effect of SSM was eliminated. However, when macrophage (M phi) and natural killer (NK) cell activities were depleted by treatment with M phi blockers (trypan blue and carrageenan) or a blocker for NK cells (anti-asialo GM1 antiserum), no alteration of the antitumor activity of SSM was observed. Therefore, T-lymphocytes, but not M phi or NK cells, were required for the expression of the antitumor efficacy of SSM. The antitumor activity of SSM was also abrogated by Lyt 1+ T-cells being depleted by treatment with monoclonal anti-Lyt 1.2 antibody, whereas the administration of monoclonal anti-Lyt 2.2 antibody had no effect on the antitumor activity. Independent of M phi, NK cells, or Lyt 2+ T-cells, Lyt 1+ T-lymphocytes appear to play an important role in the expression of the antitumor effects of SSM.

Cooperation of lymphokine(s) and macrophages in expression of antitumor activity of carboxyethylgermanium sesquioxide (Ge-132).

The administration of IFN containing sera (Ge-sera) obtained from Ge-132-treated mice (Ge-mice) or the passive transfer of macrophages (M phi) to mice bearing ascites tumors resulted in the inhibition of tumor growth. The cooperative role of Ge-sera and Ge-M phi in the display of Ge-132-antitumor activity was studied. When mice were pretreated with antimouse IFN gamma antiserum, no IFN-inducing and antitumor activities of the compound were detected. Cytotoxic activities were detected on peritoneal M phi of mice treated with Ge-sera, and passive transfer of these M phi to tumor-bearing mice resulted in the inhibition of tumor growth. When tumor-bearing mice were pretreated with substances toxic to M phi, there was no antitumor activity of Ge-sera observed. However, there was antitumor activity of Ge-sera in mice depleted of T-cells, even though the antitumor effects of the compound itself were not demonstrable in T-cell depleted mice. Therefore, a part of the antitumor activity of Ge-132 may appear to be expressed as follows: (1) Ge-132 stimulated T-cells to produce circulating lymphokine(s) which were inactivated by anti-IFN gamma treatment; (2) activated M phi were generated from resting M phi by such lymphokine(s); (3) the transplanted tumors were inhibited by these M phi.

Ability of sera from mice treated with Ge-132, an organic germanium compound, to inhibit experimental murine ascites tumours.

Sera from C57Bl/6 mice treated orally with Ge-132 exhibited antitumour activity against Ehrlich (allogeneic) and RL male 1 (syngeneic) ascites tumours in BALB/c mice. Sera obtained from mice 24 h after Ge-132 administration displayed the greatest antitumour effect and this was dose dependent. Sera prepared from mice 12, 36, or 48 h after Ge-132 treatment had no protective effect. Circulating interferon (IFN) was induced at 24 h after administration of Ge-132 but was not detected in the sera at 12, 36, or 48 h after administration. The antiviral activity of sera from Ge-132-treated mice was inactivated by treatments with trypsin, low pH, and anti-IFN gamma antiserum. The inactivated preparations of serum IFN induced by Ge-132 did not exhibit antitumour activity when administered to tumour-bearing mice. These results suggest that antitumour activity in the sera of Ge-132-treated mice may be expressed through activities of Ge-132-induced lymphokine(s), such as IFN gamma.

Importance of T-cells and macrophages in the antitumor activity of carboxyethylgermanium sesquioxide (Ge-132).

The purpose of this study was to investigate the effective mechanisms of Ge-132, an organogermanium compound with immunomodulatory activity, on experimental murine ascites tumors. The antitumor effects of Ge-132 were observed when mice inoculated with Ehrlich carcinoma (allogeneic) or RL male 1 leukemia (syngeneic) cells were treated orally. However, Ge-132 had no activity on EL-4 lymphoma (syngeneic) or Meth A fibrosarcoma (syngeneic). The antitumor activity of Ge-132 was not observed when tumor-bearing mice were treated with trypan blue, carrageenan, or monoclonal anti-Thy 1.2 antibody. However, when natural killer (NK) cells were eliminated from mice bearing RL male 1 or Ehrlich ascites tumors by treatment with anti-asialo GM1 antiserum, the antitumor activity of the compound was unchanged. This suggests that Ge-132 was effective against certain ascites tumors regardless of whether the tumor was syngeneic or allogeneic. Furthermore, its effect might be expressed through host defense mechanisms, including macrophages and/or T-cells.