Genetic control of T cell responsiveness to the Friend murine leukemia virus envelope antigen. Identification of class II loci of the H-2 as immune response genes

Mouse APOBEC3 interferes with autocatalytic cleavage of murine leukemia virus Pr180gag-pol precursor and inhibits Pr65gag processing

Mouse APOBEC3 (mA3) inhibits murine leukemia virus (MuLV) replication by a deamination-independent mechanism in which the reverse transcription is considered the main target process. However, other steps in virus replication that can be targeted by mA3 have not been examined. We have investigated the possible effect of mA3 on MuLV protease-mediated processes and found that mA3 binds both mature viral protease and Pr180gag-pol precursor polyprotein. Using replication-competent MuLVs, we also show that mA3 inhibits the processing of Pr65 Gag precursor. Furthermore, we demonstrate that the autoprocessing of Pr180gag-pol is impeded by mA3, resulting in reduced production of mature viral protease. This reduction appears to link with the above inefficient Pr65gag processing in the presence of mA3. Two major isoforms of mA3, exon 5-containing and -lacking ones, equally exhibit this antiviral activity. Importantly, physiologically expressed levels of mA3 impedes both Pr180gag-pol autocatalysis and Pr65gag processing. This blockade is independent of the deaminase activity and requires the C-terminal region of mA3. These results suggest that the above impairment of Pr180gag-pol autoprocessing may significantly contribute to the deaminase-independent antiretroviral activity exerted by mA3.

Soon after the identification of the polynucleotide cytidine deaminase APOBEC3 as a host restriction factor against vif-deficient HIV, it was noticed that deamination-independent mechanisms are involved in the inhibition of viral replication in addition to the deaminase-dependent mechanism. We previously showed that mouse APOBEC3 (mA3) physiologically restricted mouse retrovirus replication in their natural hosts without causing significant G-to-A hypermutations. Inhibition of reverse transcription is reported to be the most plausible mechanism for the deamination-independent antiretroviral function. However, it remains unknown whether the inhibition of reverse transcription is the only way to explain the whole picture of deamination-independent antiviral activity exerted by APOBEC3. Here we show that mA3 targets the autoprocessing of Pr180gag-pol polyprotein. This activity does not require the deaminase catalytic center and mainly exerted by the C-terminal half of mA3. mA3 physically interacts with murine retroviral protease and its precursor Pr180gag-pol. mA3-induced disruption of the autocatalytic Pr180gag-pol cleavage leads to a significant reduction of mature viral protease, resulting in the inhibition of Pr65gag processing to mature Gag proteins. As the Pr180gag-pol autoprocessing is necessary for the maturation of other viral enzymes including the reverse transcriptase, its inhibition by host APOBEC3 may precede the previously described impairment of reverse transcription. Our discovery may lead to the development of novel antiretroviral drugs through the future identification of detailed molecular interfaces between retroviral Gag-Pol polyprotein and APOBEC3.

Humoral immunity in the Friend retrovirus infection model

Major conceptual roadblocks impede the development of an HIV-1 vaccine that can stimulate a potent neutralizing antibody response. Animal models that support HIV-1 replication and allow for host genetic manipulation would be an ideal platform for testing various immunological hypotheses, but progress on this research front has been slow and disappointing. In contrast, many valuable concepts emerged from more than 50 years of studying the Friend retrovirus model. This was recently exemplified by the identification of an innate restriction gene, Apobec3, that could promote the retrovirus-specific neutralizing antibody response. Here we review both classical and recent data on humoral immunity against Friend retrovirus infection, and highlight the potential of this model for unraveling novel aspects of the retrovirus-specific antibody response that may guide HIV-1 vaccine development efforts.

[Molecular evolution of physiologically functioning anti-retroviral APOBEC3 deaminases]

Recent in vivo findings clearly indicate that mammalian cytidine deaminase APOBEC3 can function as a physiological restriction factor to retrotransposons and infectious retroviruses. However, some retroviruses, including primate lentiviruses, have evolved to counter their natural host's APOBEC3. To survive this arms race, primates seem to have acquired multiple copies of APOBEC3 genes. Surprisingly, however, during the process of the diversification of rodent species, as well as the human race, some ancestral individuals acquired genetic variants that reduced the protein levels of APOBEC3 expression, and these variants currently show unexpectedly wide geographic distributions. These data suggest that in the absence of a heavy burden of infectious retroviruses, high-level expression of APOBEC3 cytidine deaminase might be costly to the integrity of the host genome.

Distinct roles of CD4+ T cell subpopulations in retroviral immunity: lessons from the Friend virus mouse model

It is well established that CD4⁺ T cells play an important role in immunity to infections with retroviruses such as HIV. However, in recent years CD4⁺ T cells have been subdivided into several distinct populations that are differentially regulated and perform widely varying functions. Thus, it is important to delineate the separate roles of these subsets, which range from direct antiviral activities to potent immunosuppression. In this review, we discuss contributions from the major CD4⁺ T cell subpopulations to retroviral immunity. Fundamental concepts obtained from studies on numerous viral infections are presented along with a more detailed analysis of studies on murine Friend virus. The relevance of these studies to HIV immunology and immunotherapy is reviewed.

The frequency of HLA class I and II alleles in Turkish childhood acute leukaemia patients

In this study, blood samples were taken from 200 patients with childhood acute leukaemias, including acute lymphoblastic leukaemia (ALL) and acute myeloid leukaemia (AML), and from 100 healthy volunteers (controls). The frequency of the human leucocyte antigen (HLA)-DRB1*04 allele was significantly higher, and the frequencies of the HLA-A23 and HLA-B7 antigens were significantly lower, in patients with ALL compared with controls. Among patients with AML, the frequency of the HLA-B49 antigen and the HLA-DRB1*15 allele were significantly higher, whereas the frequencies of the HLA-A11 and HLA-B38 antigens were significantly lower compared with controls. The frequency of the HLA-DRB1*04 allele was also significantly higher in male patients with ALL and AML, whereas the HLA-DRB1*13 allele was found significantly less frequently in male AML and female ALL patients than in controls. To date, this is the only study to evaluate the associations between HLA molecules and leukaemia in a Turkish population with acute childhood leukaemia.

Mouse APOBEC3 restricts friend leukemia virus infection and pathogenesis in vivo

Several members of the apolipoprotein B mRNA-editing enzyme catalytic polypeptide-like complex 3 (APOBEC3) family in primates act as potent inhibitors of retroviral replication. However, lentiviruses have evolved mechanisms to specifically evade host APOBEC3. Likewise, murine leukemia viruses (MuLV) exclude mouse APOBEC3 from the virions and cleave virion-incorporated APOBEC3. Although the betaretrovirus mouse mammary tumor virus has been shown to be susceptible to mouse APOBEC3, it is not known if APOBEC3 has a physiological role in restricting more widely distributed and long-coevolved mouse gammaretroviruses. The pathogenicity of Friend MuLV (F-MuLV) is influenced by several host genes: some directly restrict the cell entry or integration of the virus, while others influence the host immune responses. Among the latter, the Rfv3 gene has been mapped to chromosome 15 in the vicinity of the APOBEC3 locus. Here we have shown that polymorphisms at the mouse APOBEC3 locus indeed influence F-MuLV replication and pathogenesis: the APOBEC3 alleles of F-MuLV-resistant C57BL/6 and -susceptible BALB/c mice differ in their sequences and expression levels in the hematopoietic tissues and in their abilities to restrict F-MuLV replication both in vitro and in vivo. Furthermore, upon infection with the pathogenic Friend virus complex, (BALB/c x C57BL/6)F(1) mice displayed an exacerbated erythroid cell proliferation when the mice carried a targeted disruption of the C57BL/6-derived APOBEC3 allele. These results indicate, for the first time, that mouse APOBEC3 is a physiologically functioning restriction factor to mouse gammaretroviruses.

Host genetic factors that control immune responses to retrovirus infections

Several host genes control retroviral replication and pathogenesis. These include genes that directly affect the replication of retroviruses in target cells and those that control the host immune responses to the viral antigens. Host genetic factors that affect retroviral replication and immune responses to the viral antigens have been best studied in mouse models of Friend leukemia virus (FV) infection. Several genes located within the major histocompatibility complex (MHC), along with a separate gene not linked to the MHC, influence the host immune responses to FV antigens. The latter, the Rfv3, regulates the production of virus-neutralizing antibodies, and thus affects the duration of viremia. T-cell responses to the viral epitopes are controlled by MHC class I and class II genotypes, and both CD8(+) and CD4(+) T-cells are required for spontaneous immune resistance to FV infection. When CD4(+) T-helper cells are efficiently primed with a viral epitope, however, CD8(+) T-cells are not required for immune protection against FV infection, while B cells are absolutely required. There are individuals who possess human immunodeficiency virus type 1 (HIV-1)-reactive IgA antibodies in their mucosal secretions and show strong T-cell responses to HIV-1 antigens, even though they are negative for HIV-1 genome and HIV-1-reactive serum IgG. These HIV-1-exposed but uninfected individuals rarely possess resistance-associated alleles at known AIDS-restricting loci such as CCR5Delta32. Recent genetic analyses have indicated that a large proportion of such exposed but uninfected individuals may share a common genetic background.

Mouse models of efficient and inefficient anti-tumor immunity, with emphasis on minimal residual disease and tumor escape

Tumor escape from the host immune response remains the major problem holding the development of immunotherapies for cancer. In this review, congenic mouse lines are discussed that differ dramatically in their ability to respond to tumors tested and, thereby, to survive or to succumb to the tumor and/or its metastases. This ability is under the control of either MHC class I or nontrivial MHC class II beta genes expressed in a small subpopulation of antigen-presenting cells. Two hypotheses can explain the results obtained so far: (1) emergence of tumor cell variants that escape the host immune response in morbid mice but are eliminated in survivors, and (2) tumor-induced immunosuppression, which is either efficient or not, depending on the congenic line used. It is argued that further experimentation on these congenics will allow to choose the correct hypothesis, and to characterize the mechanism(s) of elimination of minimal residual disease and prevention of tumor escape by the immune system of survivors as well as the reason(s) for its failure in morbid mice. It is also argued that the use of these models will substantially increase the chance to resolve the controversy of poor correlation of immunotherapy testing in mice with clinical results.

Genotypes at chromosome 22q12-13 are associated with HIV-1-exposed but uninfected status in Italians

OBJECTIVE

Despite multiple and repeated exposures to HIV-1, some individuals possess no detectable HIV genome and show T-cell memory responses to the viral antigens. HIV-1-reactive mucosal IgA detected in such uninfected individuals suggests their possible immune resistance against HIV. We tested if the above HIV-1-exposed but uninfected status was associated with genetic markers other than a homozygous deletion of the CCR5 gene.

METHODS

Based on our mapping in chromosome 15 of a gene controlling the production of neutralizing antibodies in a mouse retrovirus infection, we genotyped 42 HIV-1-exposed but uninfected Italians at polymorphic loci in the syntenic segment of human chromosome 22, and compared them with 49 HIV-1-infected and 47 uninfected healthy control individuals by a closed testing procedure.

RESULTS

A significant association was found between chromosome 22q12-13 genotypes and a putative dominant locus conferring anti-HIV-1 immune responses in the exposed but uninfected individuals. Distributions of linkage disequilibrium across chromosome 22 also differed between the exposed but uninfected and two other phenotypic groups.

CONCLUSIONS

The data indicated the presence of a new genetic factor associated with the HIV-1-exposed but uninfected status.

HLA system affects the age-at-onset in chronic myeloid leukemia

Chronic myeloid leukemia (CML) occurs from childhood to old age. The adult form is characterized by the presence of Philadelphia chromosome resulting from bcr/abl translocation. The BCR-ABL fusion proteins are immunogenic, and the junctional sequences show unique HLA class I and class II restriction patterns in vitro. A previous study in the west of Scotland showed an influence of several HLA genotypes on the age-at-onset of CML. In the present study, we examined the HLA-A, -B, and -DRB1/3/4/5 allele and haplotype distributions in Turkish CML patients diagnosed in a single center where they are routinely HLA-typed by PCR-SSP analysis as a preparation for stem cell transplantation. The patients were 169 subjects of age 17-60 years. The older patients were not HLA typed and missing from the study group. The age-matched control group (n = 213) was healthy blood donors from the same geographical area. HLA-B*37 showed a risk association with CML [P = 0.02; odds ratio (OR) = 5.35]. The DRB1*10 association at similar magnitude was due to its linkage disequilibrium (LD) with B*37. HLA-B*35 and DRB1*11 showed independent protective effects (P = 0.007 and 0.017; OR = 0.54 and 0.60, respectively). The protective association of DRB1*11 may be due to its involvement in the presentation of the common (b3a2) fusion gene. HLA-B*14 and DRB1*01 showed strong LD, and all 5 patients who were positive for the presumed haplotype B*14-DRB1*01 were of age 43 years old or older (P = 0.003), suggesting a delay effect. We also examined the influence of homozygosity for DRB3 (DR52) and DRB4 (DR53) haplotypes on susceptibility. As previously shown in CML and CLL, DRB4 homozygosity was a risk marker (P = 0.01; OR = 3.36), and DRB3 homozygosity was protective (P = 0.007; OR = 0.51). Despite the lack of elderly patients in the study group, the opposite accelerating (DRB4) and delaying (DRB3) effects of homozygous genotypes on the age-at-onset were evident. Besides replicating previously found associations in a different population, this study also suggested new, and probably population-specific associations in CML. The mechanisms by which the HLA system modifies susceptibility to CML are unknown, likely to include immune and nonimmune ones, and worthy of further studies.

Major histocompatibility complex heterozygote superiority during coinfection

Genes of the major histocompatibility complex (MHC) play a critical role in immune recognition, and many alleles confer susceptibility to infectious and autoimmune diseases. How these deleterious alleles persist in populations is controversial. One hypothesis postulates that MHC heterozygote superiority emerges over multiple infections because MHC-mediated resistance is generally dominant and many allele-specific susceptibilities to pathogens will be masked by the resistant allele in heterozygotes. We tested this hypothesis by using experimental coinfections with Salmonella enterica (serovar Typhimurium C5TS) and Theiler's murine encephalomyelitis virus (TMEV) in MHC-congenic mouse strains where one haplotype was resistant to Salmonella and the other was resistant to TMEV. MHC heterozygotes were superior to both homozygotes in 7 out of 8 comparisons (P = 0.0024), and the mean standardized pathogen load of heterozygotes was reduced by 41% over that of homozygotes (P = 0.01). In contrast, no heterozygote superiority was observed when the MHC haplotype combinations had similar susceptibility profiles to the two pathogens. This is the first experimental evidence for MHC heterozygote superiority against multiple pathogens, a mechanism that would contribute to the evolution of MHC diversity and explain the persistence of alleles conferring susceptibility to disease.

Crystal structure of MHC class II I-Ab in complex with a human CLIP peptide: prediction of an I-Ab peptide-binding motif

Association between the class II major histocompatibility complex (MHC) and the class II invariant chain-associated peptide (CLIP) occurs naturally as an intermediate step in the MHC class II processing pathway. Here, we report the crystal structure of the murine class II MHC molecule I-A(b) in complex with human CLIP at 2.15A resolution. The structure of I-A(b) accounts, via the peptide-binding groove's unique physicochemistry, for the distinct peptide repertoire bound by this allele. CLIP adopts a similar conformation to peptides bound by other I-A alleles, reinforcing the notion that CLIP is presented as a conventional peptide antigen. When compared to the related HLA-DR3/CLIP complex structure, the CLIP peptide displays a slightly different conformation and distinct interaction pattern with residues in I-A(b). In addition, after examining the published sequences of peptides presented by I-A(b), we discuss the possibility of predicting peptide alignment in the I-A(b) binding groove using a simple scoring matrix.

Novel role of CD8(+) T cells and major histocompatibility complex class I genes in the generation of protective CD4(+) Th1 responses during retrovirus infection in mice

CD4(+) Th1 responses to virus infections are often necessary for the development and maintenance of virus-specific CD8(+) T-cell responses. However, in the present study with Friend murine retrovirus (FV), the reverse was also found to be true. In the absence of a responder H-2(b) allele at major histocompatibility complex (MHC) class II loci, a single H-2D(b) MHC class I allele was sufficient for the development of a CD4(+) Th1 response to FV. This effect of H-2D(b) on CD4(+) T-cell responses was dependent on CD8(+) T cells, as demonstrated by depletion studies. A direct effect of CD8(+) T-cell help in the development of CD4(+) Th1 responses to FV was also shown in vaccine studies. Vaccination of nonresponder H-2(a/a) mice induced FV-specific responses of H-2D(d)-restricted CD8(+) cytotoxic T lymphocytes (CTL). Adoptive transfer of vaccine-primed CD8(+) T cells to naive H-2(a/a) mice prior to infection resulted in the generation of FV-specific CD4(+) Th1 responses. This novel helper effect of CD8(+) T cells could be an important mechanism in the development of CD4(+) Th1 responses following vaccinations that induce CD8(+) CTL responses. The ability of MHC class I genes to facilitate CD4(+) Th1 development could also be considerable evolutionary advantage by allowing a wider variety of MHC genotypes to generate protective immune responses against intracellular pathogens.

A male-specific increase in the HLA-DRB4 (DR53) frequency in high-risk and relapsed childhood ALL

Previous studies reported significant HLA-DR associations with various leukemias one of which is with HLA-DRB4 (DR53) family in male patients with childhood ALL. We have HLA-DR-typed 212 high-risk or relapsed patients with childhood (n=114) and adult (n=98) ALL and a total of 250 healthy controls (118 children, 132 adult) by PCR-SSP analysis. The members of the HLA-DRB3 (DR52) family were underrepresented in patients most significantly for HLA-DRB1*12 (P=0.0007) and HLA-DRB1*13 (P=0.0001). In childhood ALL, the protective effect of DRB3 was evident in homozygous form (P=0.001). The DRB4 marker frequency was increased in males with childhood ALL (67.4%) compared to age- and sex-matched controls (42.1%, P=0.003) and female patients (35.7%, P=0.004). Besides being a general marker for increased susceptibility to childhood ALL in males, HLA-DRB4 is over-represented in high-risk patients. These results further suggest that the HLA system is one of the components of genetic susceptibility to leukemia but mainly in childhood and in boys only.

Major histocompatibility complex class I gene controls the generation of gamma interferon-producing CD4(+) and CD8(+) T cells important for recovery from friend retrovirus-induced leukemia

Recovery from leukemia induced by Friend virus complex (FV) requires strong CD4(+) helper, CD8(+) cytotoxic T-lymphocyte, and B-cell responses. The development of these immune responses is dependent on the major histocompatibility complex (MHC) (H-2) genotype of the mouse. In H-2(b/b) mice, which spontaneously recover from FV-induced erythroleukemia, neutralization of gamma interferon (IFN-gamma) in vivo inhibited recovery, which indicated that IFN-gamma was a necessary component of the immune response to FV. Furthermore, in H-2(b/b) mice, high numbers of IFN-gamma-producing cells were detected after FV infection, whereas in H-2(a/b) mice, which have a low-recovery phenotype, only low numbers of IFN-gamma-producing cells were detected. Similarly, H-2(bm14/b) mice, which cannot recover from FV infection due to a point mutation in one allele of the H-2D(b) gene, also had low numbers of IFN-gamma-producing T cells. Surprisingly, this effect was observed for both CD8(+) and CD4(+) T cells. These findings reveal a novel influence of MHC class I genes on CD4(+) T-cell responses to viral infection. Furthermore, the influence of MHC class I genotype on the generation of both IFN-gamma-producing CD4(+) and CD8(+) T cells helps explain the major impact of the H-2D gene on recovery from FV disease.

Unravelling an HLA-DR Association in Childhood Acute Lymphoblastic Leukemia

Genetic and environmental factors play an interactive role in the development of childhood acute lymphoblastic leukemia (ALL). Since the demonstration of a major histocompatibility complex (MHC) influence on mouse leukemia in 1964, an HLA association has been considered as a possible genetic risk factor. Despite extensive efforts, however, no strong evidence comparable to the H-2k influence on mouse leukemia has been shown. The number of negative serological studies resulted in a loss of interest and consequently, no molecular HLA-DR association study has been published to date. We reconsidered the HLA-DR association in childhood ALL in 114 patients from a single center and 325 local newborn controls by polymerase chain reaction (PCR) analysis of the HLA-DRB1/3/4/5 loci. With conventional analysis, there was a moderate allelic association with the most common allele in the HLA-DR53 group, HLA-DRB1*04, in the whole group that was stronger in males (P = .0005, odds ratio = 2.9). When the other expressed HLA-DRB loci were examined, homozygosity for HLA-DRB4*01, encoding the HLA-DR53 specificity, was increased in patients (21.1%v 8.3%; odds ratio = 2.9, P = .0005). Consideration of gender showed that all of these associations were reflections of a male-specific increase in homozygosity for HLA-DRB4*01 (32.8% v 4.0%; odds ratio = 11.7, 95% confidence interval [CI] = 4.9 to 28.0; P = 3 × 10−8). This highly significant result provided the long-suspected evidence for the HLA-DR influence on the development of childhood ALL while confirming the recessive nature of the MHC influence on human leukemogenesis as in experimental models. The cross-reactivity between HLA-DR53 and H-2Ek, extensive mimicry of the immunodominant epitope of HLA-DR53 by several carcinogenic viruses, and the extra amount of DNA in the vicinity of the HLA-DRB4 gene argue for the case that HLA-DRB4*01 may be one of the genetic risk factors for childhood ALL.

Unravelling an HLA-DR Association in Childhood Acute Lymphoblastic Leukemia

Genetic and environmental factors play an interactive role in the development of childhood acute lymphoblastic leukemia (ALL). Since the demonstration of a major histocompatibility complex (MHC) influence on mouse leukemia in 1964, an HLA association has been considered as a possible genetic risk factor. Despite extensive efforts, however, no strong evidence comparable to the H-2k influence on mouse leukemia has been shown. The number of negative serological studies resulted in a loss of interest and consequently, no molecular HLA-DR association study has been published to date. We reconsidered the HLA-DR association in childhood ALL in 114 patients from a single center and 325 local newborn controls by polymerase chain reaction (PCR) analysis of the HLA-DRB1/3/4/5 loci. With conventional analysis, there was a moderate allelic association with the most common allele in the HLA-DR53 group, HLA-DRB1*04, in the whole group that was stronger in males (P = .0005, odds ratio = 2.9). When the other expressed HLA-DRB loci were examined, homozygosity for HLA-DRB4*01, encoding the HLA-DR53 specificity, was increased in patients (21.1%v 8.3%; odds ratio = 2.9, P = .0005). Consideration of gender showed that all of these associations were reflections of a male-specific increase in homozygosity for HLA-DRB4*01 (32.8% v 4.0%; odds ratio = 11.7, 95% confidence interval [CI] = 4.9 to 28.0; P = 3 × 10−8). This highly significant result provided the long-suspected evidence for the HLA-DR influence on the development of childhood ALL while confirming the recessive nature of the MHC influence on human leukemogenesis as in experimental models. The cross-reactivity between HLA-DR53 and H-2Ek, extensive mimicry of the immunodominant epitope of HLA-DR53 by several carcinogenic viruses, and the extra amount of DNA in the vicinity of the HLA-DRB4 gene argue for the case that HLA-DRB4*01 may be one of the genetic risk factors for childhood ALL.

Requirement for CD4(+) T cells in the Friend murine retrovirus neutralizing antibody response: evidence for functional T cells in genetic low-recovery mice

Recovery from infection with the Friend murine leukemia retrovirus complex (FV) requires T-helper cells and cytotoxic T cells as well as neutralizing antibodies. Several host genes, including genes of the major histocompatibility complex (H-2) and an H-2-unlinked gene, Rfv-3, influence these FV-specific immune responses. (B10.A x A/Wy)F1 mice, which have the H-2(a/a) Rfv-3(r/s) genotype, fail to mount a detectable FV-specific T-cell proliferative response but nevertheless produce FV-specific neutralizing immunoglobulin M (IgM) antibodies and can eliminate FV viremia. Thus, this IgM response, primarily influenced by the Rfv-3 gene, may be T-cell independent. To test this idea, mice were depleted of either CD4(+) or CD8(+) T-cell populations in vivo and were monitored for the effect on the neutralizing antibody response following FV infection. Surprisingly, mice in which CD4(+) cells were depleted showed undetectable FV-neutralizing antibody responses and high viremia levels compared to nondepleted or CD8-depleted animals. In addition to knocking out the FV antibody response, CD4(+) T-cell depletion reduced survival time significantly, further indicating the importance of CD4(+) T cells. These studies revealed the first evidence for a functional T-cell response following FV infection in these low-recovery mice and showed that CD4(+) T-helper cells are required for the Rfv-3-controlled FV antibody response.

Towards a genetics of anxious temperament: from mice to men

An enduring question in the study of human personality is 'to what extent and how do genetic factors influence such personality constructs as "trait anxiety"?'. We selectively survey progress and obstacles in the genetic dissection of fear-like behavior, especially as it pertains to more constitutional forms of anxiety or anxious temperament. We emphasize the selection of phenotypic dimensions and the utility of 'temperament' and personality constructs as mediating variables for psychopathology. We summarize studies on the use of mouse models of 'anxious temperament' to map genetic loci, and briefly review recent genetic association studies of related phenotypes in humans utilizing candidate genes. We suggest that further progress in genetic research on 'trait anxiety' disorders may come from the following: (i) developing alternate constructs for investigating psychiatric illness focusing on dimensional scales, mediating variables and premorbid traits; (ii) examining 'at-risk' populations for protective genetic factors influencing 'resilience' or loci providing a reduced risk of a given trait or disorder; and (iii) utilizing lower animal models as a bridge to dissect the genetic factors contributing to related human phenotypes.

Inhibitory effect of biphalin and AZT on murine Friend leukemia virus infection in vitro

Biphalin is a bivalent opioid analogue containing two tyrosine residues. We have examined the effect of biphalin's anti-retroviral potency in vitro using a murine model. Biphalin, in non-cytotoxic concentrations, suppressed in a dose-dependent fashion the replication of Friend leukemia virus (FLV) in Mus dunni cells as determined using a focus forming assay. FLV replication was substantially reduced by biphalin at 10(-4) M concentration. When biphalin was combined with 3'-azido-3'-deoxythymidine (AZT) the two acted synergistically in inhibiting FLV replication compared to either used alone. Using a reverse transcriptase (RT) assay, FLV RT levels also were noted to be reduced in the presence of biphalin. These observations indicate that biphalin possesses anti-retroviral activity in vitro, suggesting that this opioid peptide should be examined further in vivo to determine if it is a candidate for combined therapy with AZT and possibly other drugs for retrovirus infections including the human immunodeficiency virus (HIV).

Genetic control of the immune response to HIV type 1 envelope glycoprotein 120 in mice: effects of MHC and transgenic human CD4

HIV infection elicits a strong immune response to viral proteins, including broadly cross-reactive antibodies to envelope glycoprotein 120 (gp120). However, vaccination with recombinant gp120 generally produces lower titered antibodies with narrow specificity. We have examined host genes that may control the strength and breadth of the response to gp120 vaccines. Because of the complexity of the human MHC, we have focused on the response of MHC congenic mice, which share an identical genetic background, differing only in H-2 type. The antibody response to gp120 varied markedly with H-2 type. H-2a and H-2k mice gave consistently high antibody titers, while H-2s mice gave 100-fold lower titers, and H-2b mice gave low to intermediate responses. Nearly the same genetic control applied for antibodies to both unique and shared determinants and on a variety of different genetic backgrounds. Transgenic mice expressing human CD4 gave the same titers as normal H-2-matched controls. MHC-linked genetic control of the quantity and quality of antibodies indicate a requirement for T cell help in producing antibodies to unique and shared determinants of gp120.

Immunity to retroviral infection: the Friend virus model

Friend virus infection of adult immunocompetent mice is a well established model for studying genetic resistance to infection by an immunosuppressive retrovirus. This paper reviews both the genetics of immune resistance and the types of immune responses required for recovery from infection. Specific major histocompatibility complex (MHC) class I and II alleles are necessary for recovery, as is a non-MHC gene, Rfv-3, which controls virus-specific antibody responses. In concordance with these genetic requirements are immunological requirements for cytotoxic T lymphocyte, T helper, and antibody responses, each of which provides essential nonoverlapping functions. The complexity of responses necessary for recovery from Friend virus infection has implications for both immunotherapies and vaccines. For example, it is shown that successful passive antibody therapy is dependent on MHC type because of the requirement for T cell responses. For vaccines, successful immunization requires priming of both T cell and B cell responses. In vivo depletion experiments demonstrate different requirements for CD8(+) T cells depending on the vaccine used. The implications of these studies for human retroviral diseases are discussed.

The implications for childhood leukemia of infection with adenovirus

Adenoviruses have mechanisms that allow persistence in lymphoid cells and immunoevasion. They downregulate major histocompatibility complex (MHC) class I antigen expression and may trigger an autoimmune reaction against MHC class II antigens through mimicry. Adenoviral persistence could be facilitated by human leukocyte antigens A2 and DR53, thereby allowing a preleukemic clone to escape immune surveillance and progress to leukemia.

Differing T-cell requirements for recombinant retrovirus vaccines

Friend murine leukemia virus is a retrovirus complex that induces rapid erythroleukemia and immunosuppression in susceptible strains of adult mice. Using this model, we directly examined the T-cell subsets required for a protective retrovirus vaccine. Paradoxically, recovery in mice immunized with a chimeric envelope containing only T-helper (TH) and B-cell epitopes was dependent on CD8+ T cells as well as CD4+ T cells despite the fact that the vaccine contained no CD8+ cytolytic T-lymphocyte (CTL) epitopes. However, the requirement for CD8+ T cells was overcome by inclusion of additional TH and B-cell epitopes in the immunizing protein. These additional epitopes primed for more rapid production of virus-neutralizing antibody which appeared to limit virus spread sufficiently to protect even in the absence of CD8+ T cells. Inclusion of an immunodominant CTL epitope in the vaccine was not sufficient to overcome dependence on CD4+ T cells. These data suggest that TH priming is more critical for retrovirus immunity than CTL priming.

Passive immunotherapy for retroviral disease: influence of major histocompatibility complex type and T-cell responsiveness

Administration of virus-specific antibodies is known to be an effective early treatment for some viral infections. Such immunotherapy probably acts by antibody-mediated neutralization of viral infectivity and is often thought to function independently of T-cell-mediated immune responses. In the present experiments, we studied passive antibody therapy using Friend murine leukemia virus complex as a model for an immunosuppressive retroviral disease in adult mice. The results showed that antibody therapy could induce recovery from a well-established retroviral infection. However, the success of therapy was dependent on the presence of both CD4+ and CD8+ T lymphocytes. Thus, cell-mediated responses were required for recovery from infection even in the presence of therapeutic levels of antibody. The major histocompatibility type of the mice was also an important factor determining the relative success of antibody therapy in this system, but it was less critical for low-dose than for high-dose infections. Our results imply that limited T-cell responsiveness as dictated by major histocompatibility genes and/or stage of disease may have contributed to previous immunotherapy failures in AIDS patients. Possible strategies to improve the efficacy of future therapies are discussed.

Contrasting effects from a single major histocompatibility complex class II molecule (H-2E) in recovery from Friend virus leukemia

Resistance to erythroleukemia induced by infection with the Friend virus complex (FV) has been mapped to several genes residing both within and outside the murine major histocompatibility complex (MHC). MHC genes located in the A, D, and Qa/Tla regions of the murine H-2 complex have been shown to affect disease resistance through their capacity to regulate various aspects of the host immune response to viral antigens. This study establishes H-2E as the fourth MHC locus controlling immunological resistance to FV. Our investigation into the role of H-2E molecules revealed two distinct and opposite effects on recovery from Friend disease. H-2b/b mice normally lack a functional E gene product and are resistant to high doses of FV. The expression of H-2E molecules in H-2 recombinant or transgenic mice of this genotype resulted in a significant decrease in spontaneous recovery from FV-induced leukemia. In contrast, H-2E expression also appeared to influence recovery from Friend disease in a positive manner, since blocking these molecules with anti-E antibodies in vivo significantly decreased recovery from Friend disease. The data indicate that the positive effects of H-2E molecules derive from their function as restriction elements for helper T-cell recognition of the viral envelope glycoprotein, and we postulate that the negative effects are due to H-2E-dependent deletion in the T-cell repertoire during development.

Production and characterization of new monoclonal antibodies that distinguish subsets of mink lymphoid cells

Several hybridoma clones that produce monoclonal antibodies (MAbs) reacting with subpopulations of mink lymphoid cells were established. Two of the MAbs, MTS-4.3 and MTS-9.3, reacted with relatively small populations of surface immunoglobulin (Ig)-negative (Ig-) lymphocytes. MTS-4.3+ and MTS-9.3+ cells were distributed in the thymic cortex and medulla, paracortical areas of lymph nodes, and periarterial lymphoid sheaths of the spleen, indicating that these MAbs identify T lymphocytes. Another MAb, MTB-5.6, reacted with a large proportion of surface Ig+ lymph node cells, but not with surface Ig- cells. In immunohistochemistry this MAb stained dendritic epithelial cells of thymic cortex, large polygonal cells of thymic medulla, a large proportion of lymphocytes in the mantle zone of lymphoid follicles, dendritic-shaped cells of paracortical area, and some lymphocytes and macrophage-like cells of medullary cords and sinuses of lymph nodes. The expression of the cell-surface antigen reacting with MTB-5.6 on Ig+ lymph node cells was increased after concanavalin A stimulation. These new reagents may be useful to analyze cellular basis of the abnormal immune responses observed in Aleutian mink disease, a classical model of human autoimmune diseases.

Distinct H-2 complex control of mortality, and immune responses to tuberculosis infection in virgin and BCG-vaccinated mice

We have studied the impact of distinct haplotypes and of different alleles at specific H-2 loci on: (i) the susceptibility to lethal form of experimental tuberculosis; (ii) the level of DTH to mycobacterial antigens; (iii) the efficacy of vaccination with bacille Calmette-Guérin (BCG); and (iv) the IgG production and T cell proliferative response to H37Rv antigens. On the basis of median survival time (MST) following primary inoculation with lethal dose of Mycobacterium tuberculosis, susceptibility to infection associated with I-Ab and Db alleles, host resistance associated with I-Ak and Dd alleles. Mice bearing a disease-resistant phenotype also developed a vigorous DTH response. Vaccination with BCG before H37Rv infection significantly prolonged the survival time of both resistant and susceptible animals, except in B10.M (H-2f) mice. The latter exhibited intermediate resistance to infection before but slight decrease in the MST following a high-dose BCG vaccination. Distinct H-2 regulation of susceptibility to lethal infection and of BCG vaccination efficacy was confirmed in another relatively resistant H-2f-bearing strain A.CA, in which mortality occurred more rapidly in vaccinated compared with primarily infected animals. The expression of the H-2f haplotype was associated with a low DTH response to tuberculin following vaccination and subsequent lethal infection. The lack of BCG protection against Myco, tuberculosis challenge in B10.M mice associated with the high titre of specific IgG. In addition, these mice exhibited a unique ability to respond to 65-kD antigen by both IgG synthesis and T cell proliferation.

Multiplicity of virus-encoded helper T-cell epitopes expressed on FBL-3 tumor cells

To identify retroviral antigenic determinants recognized by CD4+ T helper cells during tumor rejection, we established four noncytolytic, helper-type, CD4+ T-cell clones by limiting dilution cultures of mixed lymphocyte-tumor cultures from mice immune to a Friend virus-induced tumor, FBL-3. Among these, three T helper cell clones were isolated from C57BL/6 mice and the fourth was isolated from a (BALB/c x C57BL/6)F1 mouse. All these clones proliferated in response to the immunizing FBL-3 tumor cells in a major histocompatibility complex class II-restricted manner. Each clone expressed a distinct T-cell receptor with a characteristic combination of alpha and beta chains. The localization of helper T-cell determinants on viral proteins was analyzed with recombinant vaccinia viruses expressing Friend murine leukemia virus (F-MuLV) gag or env genes or shorter fragments of the env gene. Epitopes recognized by these T-cell clones were mapped to at least two distinct portions in the env region of the F-MuLV genome. These epitopes were identified more precisely with synthetic peptides derived from the F-MuLV envelope protein sequence. One of these epitopes was common to Friend and Moloney MuLVs and was located in the N-terminal region of the gp70 glycoprotein at amino acids 122 to 141. The second epitope, which was recognized in the context of hybrid I-Eb/d major histocompatibility complex class II molecule, was located close to the C-terminal end of gp70 at amino acids 462 to 479. In addition, a possible third epitope was located in the N-terminal half of the gp70 sequence and differed from the first epitope in that it was not cross-reactive with the Moloney MuLV envelope protein.

Protection against Friend retrovirus-induced leukemia by recombinant vaccinia viruses expressing the gag gene

High sequence variability in the envelope gene of human immunodeficiency virus has provoked interest in nonenvelope antigens as potential immunogens against retrovirus infection. However, the role of core protein antigens encoded by the gag gene in protective immunity against retroviruses is unclear. By using recombinant vaccinia viruses expressing the Friend murine leukemia helper virus (F-MuLV) gag gene, we could prime CD4+ T-helper cells and protectively immunize susceptible strains of mice against Friend retrovirus infection. Recovery from leukemic splenomegaly developed more slowly after immunization with vaccinia virus-F-MuLV gag than with vaccinia virus-F-MuLV env; however, genetic nonresponders to the envelope protein could be partially protected with Gag vaccines. Class switching of F-MuLV-neutralizing antibodies from immunoglobulin M to immunoglobulin G after challenge with Friend virus complex was facilitated in mice immunized with the Gag antigen. Sequential deletion of the gag gene revealed that the major protective epitope was located on the N-terminal hydrophobic protein p15.

Detailed mapping of the Rfv-1 gene that influences spontaneous recovery from Friend retrovirus-induced leukaemia

Using H-2 recombinant and mutant mice, the Rfv-1 gene influencing spontaneous recovery from Friend retrovirus (FV)-induced leukaemia was mapped in the D locus. Two Db alleles were required for full recovery, and a single Dd transgene did not convey increased susceptibility to FV in the presence of homozygous Db/b genotype. The results suggest that an increase in the expression of Db may lead to more effective stimulation of FV-specific CTL.

Role and specificity of T-cell subsets in spontaneous recovery from Friend virus-induced leukemia in mice

Spontaneous recovery from Friend virus complex-induced leukemic splenomegaly in H-2Db/b mice correlated with the appearance of Friend virus complex-specific cytotoxic T lymphocytes (CTL) detectable directly in spleen cell populations. By testing CTL on target cells containing expression vectors encoding individual retroviral structural proteins, the main viral protein recognized was shown to be the Friend murine leukemia helper virus envelope glycoprotein. In vivo depletion of CD8-positive T cells drastically reduced the incidence of recovery, providing direct evidence for the role of CD8-positive CTL in the spontaneous recovery process. In vivo depletion of CD4-positive cells had little effect on the early stages of recovery but did cause a marked reduction in the final incidence of recovery at 60 to 90 days. Thus, CD8-positive cells were required for the initiation of the recovery process, whereas CD4-positive cells appeared to be required for maintenance of the recovered status.

Elucidation of mode of retroviral-inhibitory effects of imexon through use of immune competent and severe combined immune deficiency (SCID) mice

Mice infected with various tumor retroviruses have been used as models for evaluating therapeutic substances for the treatment of some cancers, and more recently, for human immunodeficiency virus (HIV) infection, the causative agent of acquired immune deficiency syndrome (AIDS). Consequently, there is a need to determine the ability of biological response modifiers (BRMs) to specifically reduce virus-infected cells, as compared to their non-specific anti-proliferative effects. To address this need, a BRM, imexon, was evaluated in this study using three strains of mice having different Friend virus (FV)-specific immunological capabilities. The first strain, (B10.A x A/WySn)F1, was genetically capable of producing FV-specific neutralization and cytotoxic antibodies, the second, Balb/c, was not, and the third, SCID mice, lacked functional T and B cell immunity. Imexon treatment reduced virally-induced splenomegaly in all 3 strains; however, the concentration of splenic viral infectious centers (IC) were not affected. Since imexon was efficacious in reducing splenomegaly in SCID mice, the mode of action was concluded to not require functional T or B cell immunity. The observation that imexon did not affect splenic IC titers also suggested that imexon did not specifically eliminate virally infected cells, but may have functioned by other mechanisms. This study also demonstrated the use of various mouse strains as a strategy for delineating the modes of action of BRMs against murine retroviral infections.

Use of low toxicity adjuvants and killed virus to induce protective immunity against the Friend murine leukaemia retrovirus-induced disease

Low toxic and synthetic adjuvants were investigated in the induction of protective immunity against Friend murine retrovirus-induced erythroleukaemia by immunization with inactivated Friend murine leukaemia helper virus (F-MuLV). 6-O-(2-tetradecyl-hexadecanoyl)-N-acetylmuramyl-L-alanyl-D-isoglutamine (B30-MDP) showed a significant enhancement of the protective immunity against Friend virus-induced erythroleukaemia not only in H-2a/b mice known to make good immune responses to F-MuLV envelope, but also in H-2a/a mice which are usually unable to respond to F-MuLV envelope protein. Another analogue of N-acetylmuramyl-D-isoglutamine (MDP), N alpha-acetylmuramyl-L-alanyl-D-isoglutaminyl-N epsilon-stearoyl-L-lysine [MDP-Lys(L18)], which has been shown to enhance non-specific protective activity against bacterial and viral infections, however, showed no adjuvant activity in the present system. A combined adjuvant of the synthesized mycobacterial cord factor, trehalose dimycolate (TDM) and detoxified bacterial endotoxin, monophosphoryl lipid A from Salmonella minnesota, gave good protection which was comparable to complete Freund's adjuvant in both H-2a/b and H-2a/a mice.

Host genes conferring resistance to a central nervous system disease induced by a polytropic recombinant Friend murine retrovirus

Infection of certain strains of mice with the ecotropic Friend murine leukemia virus results in the generation of recombinant polytropic mink cell focus-inducing viruses and the development of erythroleukemia. We isolated a Friend mink cell focus-inducing virus (F-MCF-98D) from a Friend murine leukemia virus-infected BALB/c mouse which caused primarily a neurological disease as well as a low incidence of leukemia in susceptible IRW mice. Through genetic studies with the resistant C57BL/10 strain, we identified two genes which correlated with restricted viral replication and resistance to the development of disease caused by F-MCF-98D. One gene correlated with the expression of an endogenous gp70 linked to the Rmcf gene and might act by viral interference. The mechanism of action of the second gene was less clear, but it appeared to be associated with development of an antiviral antibody response.

In vivo generation of antigenic variants of murine retroviruses

Inoculation of adult BALB/c-H-2k (BALB.K) mice with both Gross murine leukemia virus (GV) and a biological clone derived from this virus resulted in the recovery of variant viruses which differ from GV with respect to the expression of specific epitopes associated with the env gene product, gp70. The loss of these epitopes correlated with the failure of antiserum raised in BALB.K mice against GV to neutralize variant virus although this antiserum neutralized GV. In contrast, BALB/c-H-2b (BALB.B) mice, immunized with GV, produced antibodies which neutralized both GV and the variant virus, indicating that BALB.B mice respond to epitopes distinct from those recognized by BALB.K mice. These results suggest that the selection of variant viruses resulting from in vivo passage may be related to the immunoselective pressures exerted in mice which express particular alleles of certain major histocompatibility complex (MHC)-linked genes.