Analysis of MHC class I expression in equine trophoblast cells using in situ hybridization.

Down-regulation of major histocompatibility complex (MHC) genes by trophoblast cells is considered to be a primary mechanism preventing maternal immune rejection of the fetal-placental unit in mammalian pregnancy by rendering these cells, which form the primary barrier between mother and fetus, relatively non-antigenic. In situ hybridization with probes encoding human and horse MHC class I genes was used to characterize the pattern of MHC class I mRNA expression in the various forms of horse trophoblast. Strong hybridization signals were observed in the invasive trophoblast cells of chorionic girdle tissue. In contrast, no hybridization signal specific for MHC class I mRNA transcripts was observed in the descendent endometrial cup trophoblast cells. In the non-invasive trophoblast cells of the allantochorion, no hybridization signals specific for horse MHC class I mRNA transcripts were consistently detected. In parallel to the in vivo results, strong hybridization signals were observed in the small, mononuclear cells present in chorionic girdle cell explant cultures, but not in the population of large binucleate cells corresponding to endometrial cup cells. The results obtained using in situ hybridization are consistent with the hypothesis that expression of MHC class I genes may be controlled at the transcriptional level in horse invasive and non-invasive trophoblast cells, and suggest that down-regulation of MHC class I antigen expression in endometrial cup cells may be accomplished by the same mechanisms in vivo and in vitro.

Feline aminopeptidase N is a receptor for all group I coronaviruses.

Human coronavirus HCV-229E and porcine transmissible gastroenteritis virus (TGEV), both members of coronavirus group I, use aminopeptidase N (APN) as their cellular receptors. These viruses show marked species specificity in receptor utilization as they can only use APN of their respective species to initiate virus infection. Feline and canine coronaviruses are also group I coronaviruses. To determine whether feline APN could serve as a receptor for feline coronaviruses (FCoVs), we cloned the cDNA encoding feline APN (fAPN) by PCR from feline cells and stably expressed it in FCoV-resistant mouse or hamster cells. These became susceptible to infection with either of several biotypes of FCoVs. The expression of recombinant fAPN also made hamster and mouse cells susceptible to infection with other group I coronaviruses, including several canine coronavirus strains, transmissible gastroenteritis virus (TGEV), and human coronavirus HCV-229E. Thus, fAPN served as a functional receptor for each of these coronaviruses in group I. As expected, fAPN could not serve as a receptor for mouse hepatitis virus (MHV), a group II coronavirus which uses murine biliary glycoproteins as receptors. Thus, fAPN acts as a common receptor for coronaviruses in group I, in marked contrast to human and porcine APN glycoproteins which serve as receptors only for human and porcine coronaviruses, respectively. These observations suggest that cats could serve as a "mixing vessel" in which simultaneous infection with several group I coronaviruses could lead to recombination of viral genomes.

Virus-receptor interactions in the enteric tract. Virus-receptor interactions.

Expression of specific virus receptors on the surface of intestinal epithelial cells or M cells can determine whether or not a animal is susceptible to infection with an enterotropic virus. Receptors for many animal viruses have been identified. The specificity of virus-receptor interactions clearly affects the species specificity of virus infection, and in some instances may be an important determinant of viral tissue tropism. In this paper, the specificity of coronavirus-receptor interactions is summarized. Porcine and human coronaviruses utilize aminopeptidase N as their receptors, but in a species-specific manner. Mouse hepatitis virus uses several rodent glycoproteins in the carcinoembryonic antigen family as receptors. In addition, some coronaviruses can interact with carbohydrate moieties on the cell surface. Understanding the molecular mechanisms of virus-receptor interactions may lead to development of novel strategies for the control of enteric viral diseases.

Duration of immunity in cats after vaccination or naturally acquired infection.

The necessity for cats to be vaccinated annually against common pathogens has been questioned because sarcomas have infrequently been reported at the injection site. However, with few exceptions, the duration of immunity induced by vaccination or infection is uncertain, and there may therefore be a risk associated with a decision not to revaccinate. This article reviews the information available about the duration of immunity induced by vaccination or infection in cats, and reveals many shortcomings that make blanket recommendations impossible. Each vaccine must be considered individually.

Duration of immunity in dogs after vaccination or naturally acquired infection.

This paper reviews current scientific information about the duration of immunity induced in dogs by infection or vaccination. It describes the shortcomings of the methods used to measure the immune responses of dogs, and explains the need for basic studies on the nature of protective humoral and cellular responses, and standardised assays for the long-term duration of immunity to pathogens other than rabies. The information is inadequate to warrant uniform recommendations on the ideal intervals for vaccination; each vaccine must be evaluated on the basis of its own merits and the characteristics of the disease it is intended to guard against.

Cells of human aminopeptidase N (CD13) transgenic mice are infected by human coronavirus-229E in vitro, but not in vivo.

Aminopeptidase N, or CD13, is a receptor for serologically related coronaviruses of humans, pigs, and cats. A mouse line transgenic for the receptor of human coronavirus-229E (HCoV-229E) was created using human APN (hAPN) cDNA driven by a hAPN promoter. hAPN-transgenic mice expressed hAPN mRNA in the kidney, small intestine, liver, and lung. hAPN protein was specifically expressed on epithelial cells of the proximal convoluted renal tubules, bronchi, alveolar sacs, and intestinal villi. The hAPN expression pattern within transgenic mouse tissues matched that of mouse APN and was similar in mice heterozygous or homozygous for the transgene. Primary embryonic cells and bone marrow dendritic cells derived from hAPN-transgenic mice also expressed hAPN protein. Although hAPN-transgenic mice were resistant to HCoV-229E in vivo, primary embryonic cells and bone marrow dendritic cells were infected in vitro. hAPN-transgenic mice are valuable as a source of primary mouse cells expressing hAPN. This hAPN-transgenic line will also be used for crossbreeding experiments with other knockout, immune deficient, or transgenic mice to identify factors, in addition to hAPN, that are required for HCoV-229E infection.

Feline aminopeptidase N serves as a receptor for feline, canine, porcine, and human coronaviruses in serogroup I.

Two members of coronavirus serogroup I, human respiratory coronavirus HCV-229E and porcine transmissible gastroenteritis virus (TGEV), use aminopeptidase N (APN) as their cellular receptors. These viruses show marked species specificity in receptor utilization, as HCV-229E can utilize human but not porcine APN, while TGEV can utilize porcine but not human APN. To determine whether feline APN could serve as a receptor for two feline coronaviruses in serogroup I, feline infectious peritonitis virus (FIPV) and feline enteric coronavirus (FeCV), we cloned the cDNA encoding feline APN (fAPN) by PCR from cDNA isolated from a feline cell line and stably expressed it in FIPV- and FeCV-resistant mouse and hamster cells. The predicted amino acid sequence of fAPN shows 78 and 77% identity with human and porcine APN, respectively. When inoculated with either of two biologically different strains of FIPV or with FeCV, fAPN-transfected mouse and hamster cells became infected and viral antigens developed in the cytoplasm. Infectious FIPV was released from hamster cells stably transfected with fAPN. The fAPN-transfected mouse and hamster cells were challenged with other coronaviruses in serogroup I including canine coronavirus, porcine coronavirus TGEV, and human coronavirus HCV-229E. In addition to serving as a receptor for the feline coronaviruses, fAPN also served as a functional receptor for each of these serogroup I coronaviruses as shown by development of viral antigens in the cytoplasm of infected mouse or hamster cells stably transfected with fAPN. In contrast, fAPN did not serve as a functional receptor for mouse hepatitis virus (MHV-A59), which is in serogroup II and utilizes mouse biliary glycoprotein receptors unrelated to APN. Thus, fAPN serves as a receptor for a much broader range of group I coronaviruses than human and porcine APNs. The human, porcine, and canine coronaviruses in serogroup I that are able to use fAPN as a receptor have previously been shown to infect cats without causing disease. Therefore, host factors in addition to receptor specificity apparently affect the virulence and transmissibility of nonfeline serogroup I coronaviruses in the cat.

Analysis of the cell and erythrocyte binding activities of the dimple and canyon regions of the canine parvovirus capsid.

Canine parvovirus (CPV) binds to a number of cell and erythrocyte receptors, some of which are involved in cell infection, while others are used for other viral functions. Little is known about the regions of the virus capsid which bind to the cell receptors. CPV binds sialic acid through a region within or adjacent to the dimple on the surface of the capsid (Barbis, D. P., Chang, S-F., and Parrish, C. R., 1992, Virology 191, 301-308). In order to map the sialic acid binding site in more detail and to examine other regions of the capsid for cell receptor binding, a variety of mutant capsids were analyzed which had changes in two depressions within the surface of the capsid--the "canyon" and "dimple." In most cases recombinant VP1 and VP2 proteins were stably expressed together in canine A72 cells from a plasmid expression vector. The purified empty capsids were tested for their ability to bind sialic acid and thereby hemagglutinate (HA) erythrocytes and for binding to permissive host cells. In addition, the ability of neutralizing monoclonal antibodies to block cell attachment was also examined. Mutations of amino acids on a wall of the dimple eliminated or severely decreased HA. Changing various residues within the canyon had no effect on binding to either sialic acids or other receptors on feline lymphoblastoid cells, suggesting that the canyon is not the site of cell receptor attachment. Neutralizing monoclonal antibodies against both major antigenic determinants had variable effects on cell binding, but no consistent inhibition of binding was observed by antibodies directed against either of those two major antigenic determinants of the capsid.