Restored humoral immune response to influenza vaccination in HIV-infected adults treated with highly active antiretroviral therapy

BACKGROUND

Highly active antiretroviral therapy (HAART) effectively suppresses replication of HIV and is accompanied by an increase in CD4+ T lymphocytes. Whether the increase in CD4+ T lymphocytes in the blood is a reflection of a reconstitution of the immune functions is unknown. We investigated the recovery of the humoral immune response during HAART after immunization with T-cell-dependent influenza vaccine.

METHODS

Forty-one men and three women infected with HIV and treated with HAART, and 15 healthy hospital staff members were immunized with trivalent influenza subunit vaccine. Antibody titres were determined by haemagglutination inhibiting assay in sera obtained before and 30 days after immunization. Lymphocyte subsets were determined in blood samples taken at the time of vaccination.

RESULTS

In all HIV-infected individuals, treatment with HAART caused a median reduction of 2.3 log10 in HIV-1 load. The median increase of CD4+ T lymphocytes after initiation of HAART was 170 x 10(6)/l. The antibody response to influenza antigens was proportional to the number of memory CD4+ T lymphocytes in the blood at the time of vaccination. When a group of patients and healthy controls with approximately similar CD4+ T-lymphocyte counts were considered, the antibody titres after vaccination for influenza strain H1N1 and influenza B did not differ between patients and controls (P=0.12). Vaccination of patients with a CD4+ T-lymphocyte count of < 200 x 10(6)/l (mean 85 x 10(6)/l) before the start of HAART and with a mean of 282 x 10(6)/l CD4+ T lymphocytes at the time of vaccination as a result of HAART, demonstrated a substantial antibody response whereas patients with a CD4+ T lymphocyte count of < 200 x 10(6)/l (mean 56 x 10(6)/l) not treated with HAART (historical controls), and vaccinated with a similar influenza vaccine, failed to induce an antibody response.

CONCLUSION

The present findings demonstrate a recovery of the humoral immune response to influenza antigens in HIV-infected individuals treated with HAART. This indicates that functional improvement of antigen specific CD4+ T helper cell reponses occurs.

Use of recombinant nucleoproteins in enzyme-linked immunosorbent assays for detection of virus-specific immunoglobulin A (IgA) and IgG antibodies in influenza virus A- or B-infected patients

The nucleoprotein genes of influenza virus A/Netherlands/018/94 (H3N2) and influenza virus B/Harbin/7/94 were cloned into the bacterial expression vector pMalC to yield highly purified recombinant influenza virus A and B nucleoproteins. With these recombinant influenza nucleoproteins, enzyme-linked immunosorbent assays (ELISAs) were developed for the detection of influenza virus A- and B-specific immunoglobulin A (IgA) and IgG serum antibodies. Serum samples were collected at consecutive time points after the onset of clinical symptoms from patients with confirmed influenza virus A or B infections. Nucleoprotein-specific IgA antibodies were detected in 41.2% of influenza virus A-infected patients and in 66. 7% of influenza virus B-infected patients on day 6 after the onset of clinical symptoms. In serum samples taken on day 21 (influenza virus A-infected patients) or day 28 (influenza virus B-infected patients), nucleoprotein-specific IgA antibodies could be detected in 58.8 and 58.3% of influenza virus A- and B-infected patients, respectively. At the same time, IgG antibody rises were detected in 88.2% of influenza virus A-infected patients and in 95.8% of influenza virus B-infected patients. On comparison, hemagglutination inhibition assays detected antibody titer rises in 81.3 and 72.7% of patients infected with influenza viruses A and B, respectively. In contrast to the detection of nucleoprotein-specific IgG antibodies or hemagglutination-inhibiting antibodies, the detection of nucleoprotein-specific IgA antibodies does not require paired serum samples and therefore can be considered an attractive alternative for the rapid serological diagnosis of influenza.

[The influenza season 1997/'98 and the vaccine composition for 1998/'99]

The 1997/'98 influenza season in the Netherlands was marked by influenza A/H3N2 activity which never reached a true epidemic level. There was no real peak activity either but a prolonged period of increased activity of approximately eight weeks with a maximum in week 13, when sentinel physicians reported 16.6 cases of influenza-like illness per 10,000 inhabitants. It was not until week 18 of 1998 that the influenza activity declined to baseline levels. During the season, almost exclusively influenza A/H3N2 viruses were isolated, of which the majority resembled the new strain influenza A/Sydney/5/97 (H3N2). Further analysis of these variant viruses revealed that, although there was some cross-reactivity with the vaccine strain (A/Nanchang/933/95), no optimal protection could be expected to be induced by the vaccine. Antigenic characterisation of the sporadic influenza A/H1N1 and influenza B viruses showed that these were related to the vaccine strains. As a result of these findings, the World Health Organization (WHO) recommended to change the H3N2 strain in the influenza vaccine for the season 1998/'99 to an influenza A/Sydney/5/97(H3N2)-like strain. Based on epidemiological data from other countries, it was also decided to change the influenza A/H1N1 component to an influenza A/Beijing/262/95 (H1N1)-like strain.

Accelerated viraemia in cats vaccinated with fixed autologous FIV-infected cells

We have vaccinated cats with fixed autologous FIV infected PBMC to determine whether autologous presentation of antigen is capable of inducing a protective immune response against homologous challenge. To this end autologous PBMC were infected with a FIV molecular clone (19k1). When infection was established, cells were inactivated by dialysis against paraformaldehyde. Upon vaccination, cats developed a virus specific immune response as measured by ELISA against the Gag protein of FIV. No antibodies against the envelope protein were detected with a peptide ELISA. Virus neutralizing antibodies however could be detected with a neutralization assay based on infection of CrFK cells, but not in an assay based on infection of primary T-cells. Although vaccination led to the induction of these virus-specific immune responses, vaccinated cats were not protected against homologous challenge but showed an accelerated viraemia upon infection. This was shown both by PCR and cell-associated viral load. The possible mechanisms underlying this observation are discussed in this paper.

Comparison of RNA hybridization, hemagglutination assay, titration of infectious virus and immunofluorescence as methods for monitoring influenza virus replication in vitro

Rapid and sensitive methods for the monitoring of influenza virus replication in vitro are needed to address several research questions. Four methods based on different principles were compared: the hemagglutination (HA) assay, the measurement of virus infectivity titers in culture supernatants, the enumeration of infected cells by immunofluorescence and RNA hybridization techniques using digoxigenin (DIG) labeled RNA probes. To this end, MDCK cells were infected at different multiplicities of infection (moi) with a recent influenza A virus (A/Netherlands/18/94 H3N2) and the kinetics of virus replication were monitored with these four assays. At high moi, virus released into the culture supernatant of infected cells was detected by the HA assay 12 h post infection, whereas at lower moi (< or = 0.01) the first HA activity was not detected before 24 h post infection. The measurement of infectious viruses in the culture supernatant proved to be more sensitive, since 4-12 h post infection newly produced virus was detected depending on the moi used. This finding was in agreement with results obtained by the immunofluorescence assay using an antibody preparation specific for the nucleoprotein: single infected cells could be detected as early as 4 h post infection. At this time point, positive signals were also obtained when mRNA/cRNA specific hybridization was carried out for the NP gene segment, but not for viral NP RNA or RNA specific for the hemagglutinin, which were only detected at later time points after infection. Thus, besides direct measurement of infectious virus and immunofluorescence, RNA hybridization proved to be a sensitive assay for monitoring influenza virus replication in vitro.

Human influenza virus A/HongKong/156/97 (H5N1) infection

Introduction of influenza viruses with gene segments of avian origin into the human population may result in the emergence of new pathogenic human influenza viruses. The recent infection of a 3-year-old boy with an influenza A (H5N1) virus of avian origin can be considered as an example of such an event. However, this virus, influenza A/Hong Kong/156/97 (H5N1) and the 17 additional H5N1 viruses isolated from humans by the end of 1997 lack the ability to spread efficiently amongst humans and therefore have limited pandemic potential. However, the possibility of reassortment of these viruses with currently circulating human viruses illustrates the need for pandemic preparedness.

Induction of virus-specific immunity by iscoms

Immune stimulating complexes (iscoms) are 30-40 nm cage-like structures, which consist of glycoside molecules of the adjuvant Quil A, cholesterol and phospholipids in which antigen can be integrated. Even in the presence of pre-existing antibodies they may function as a potent adjuvant system by inducing high systemic and local long-lasting antibody and T cell responses. An additional advantage is their capacity to induce MHC class I restricted CD8+ CTL responses. This combination will in most cases, when the relevant antigens are incorporated, lead to adequate protection against virus infection or disease. An overview is given of the present data available on the use of iscoms in viral systems, and some relevant examples are discussed to highlight general principles in the use of iscoms as an adjuvant system.

Human influenza A H5N1 virus related to a highly pathogenic avian influenza virus

BACKGROUND

In May, 1997, a 3-year-old boy in Hong Kong was admitted to the hospital and subsequently died from influenza pneumonia, acute respiratory distress syndrome, Reye's syndrome, multiorgan failure, and disseminated intravascular coagulation. An influenza A H5N1 virus was isolated from a tracheal aspirate of the boy. Preceding this incident, avian influenza outbreaks of high mortality were reported from three chicken farms in Hong Kong, and the virus involved was also found to be of the H5 subtype.

METHODS

We carried out an antigenic and molecular comparison of the influenza A H5N1 virus isolated from the boy with one of the viruses isolated from outbreaks of avian influenza by haemagglutination-inhibition and neuraminidase-inhibition assays and nucleotide sequence analysis.

FINDINGS

Differences were observed in the antigenic reactivities of the viruses by the haemagglutination-inhibition assay. However, nucleotide sequence analysis of all gene segments revealed that the human virus A/Hong Kong/156/97 was genetically closely related to the avian A/chicken/Hong Kong/258/97.

INTERPRETATION

Although direct contact between the sick child and affected chickens has not been established, our results suggest transmission of the virus from infected chickens to the child without another intermediate mammalian host acting as a "mixing vessel". This event illustrates the importance of intensive global influenza surveillance.

Feline immunodeficiency virus subunit vaccines that induce virus neutralising antibodies but no protection against challenge infection

Three experimental vaccines against feline immunodeficiency virus (FIV), all based on viral antigens presented via immune stimulating complexes (iscoms), were tested for their capacity to induce protection in cats from FIV infection. The respective vaccines consisted of FIV propagated in Crandell feline kidney (CrFK) cells (FIV-iscoms); FIV-iscoms spiked with recombinant vaccinia virus expressed FIV envelope glycoprotein incorporated into iscoms (FIV-iscoms + vGR657x15-iscoms) and vGR657x15-iscoms spiked with recombinant FIV Gag protein incorporated into iscoms (vGR657x15-iscoms + FIV-Gag-iscoms). Simian immunodeficiency virus envelope glycoprotein incorporated into iscoms, iscoms prepared with uninfected CrFK cells, and PBS served as controls. All cats vaccinated with vGR657x15-iscoms combined with FIV-iscoms or FIV-Gag-iscoms developed Env-specific plasma antibody responses. These antibodies neutralised FIV infection in CrFK cells, but failed to neutralise FIV infection in primary feline thymocytes. FIV-iscoms induced poor Env-specific responses and only one out of six cats developed antibodies that neutralised FIV in the CrFK cell based assay. Four weeks after challenge all cats proved to be infected, showing that none of the vaccine preparations provided protection. In contrast, 2 weeks after infection, virus infected peripheral blood mononuclear cells were only observed in cats vaccinated with FIV-iscoms + vGR657x15-iscoms or CrFK-iscoms and to a lesser extent in cats vaccinated with FIV-iscoms and vGR657x15-iscoms + FIV-Gag-iscoms, but not in cats vaccinated with SIV-iscoms or PBS. The differences found in cell associated virus loads amongst the respective groups are discussed in the light of antibody mediated enhancement of infectivity and protective effects provided by Gag-specific T cell responses.

[Influenza in the 1996/'97 season; vaccine composition for the 1997/'98 season]

The first indication of flu activity in the Netherlands in the 1996/'97 season was the isolation of an A/H3N2 influenza virus in week 48 of 1996. In subsequent weeks influenza viruses were isolated sporadically. The clinical influenza activity increased from week I of 1997 and reached its peak in week 4 of 1997. Simultaneously with the increase of clinical influenza activity, an increasing number of influenza viruses were isolated. The epidemic had a relatively small extent. Initially, A/H3N2 influenza viruses were predominant, but in the second half of the epidemic an increasing number of influenza B viruses were isolated as well. The A/H3N2 viruses were antigenically fairly strongly distinct from the variants prevalent in the preceding years. This season influenza A/HINI viruses did not play a significant role and only one virus of this subtype was isolated. All influenza A/H3N2, A/HINI and B viruses isolated were antigenically similar to the vaccine strains.

Modulation of primary human immunodeficiency virus type 1 envelope glycoprotein-mediated entry by human antibodies

Recently, we and others have shown that the interaction between envelope specific antibodies and primary human immunodeficiency virus type 1 (HIV-1) isolates may result in either inhibition or enhancement of virus entry. The outcome proved to be determined by the virus isolate rather than by the specificity of the antiserum used. To study the mechanism underlying this phenomenon, a series of HIV-1 envelope glycoproteins from closely related primary virus isolates of different syncytium inducing phenotypes, together with chimeras of these proteins, were tested in an envelope trans-complementation assay for their sensitivity to either antibody mediated inhibition or enhancement of HIV-1 entry. Based on the observation that, in contrast to the inhibition of HIV-1 entry, antibody mediated enhancement was not temperature dependent and could not be mediated by F(ab) fragments, we concluded that the mechanisms underlying these phenomena are different and that antibody mediated enhancement of HIV-1 entry is largely if not exclusively mediated by HIV-1 glycoprotein cross-linking. The susceptibility of the envelope glycoprotein chimeric viruses to neutralization or enhancement of infectivity proved to be primarily determined by the configuration of the V3 loop, and the affinity of the antibodies to monomeric HIV-1 gp 160 molecules, proved to be of quantitative importance only. One human monoclonal antibody directed against gp41 (IAM 2F5) inhibited entry of all the viruses studied, irrespective of their phenotype, and directly proportional to its affinity to monomeric HIV-1 gp 160.

Induction of protective immunity against influenza virus in a macaque model: comparison of conventional and iscom vaccines

Cynomolgus macaque monkeys (Macaca fascicularis) were immunized twice intramuscularly, either with a conventional non-adjuvanted subunit vaccine or with a candidate immune-stimulating complex (iscom) vaccine, each containing 10 micrograms envelope glycoprotein of a recent human influenza A(H3N2) virus (A/Netherlands/18/94). In contrast to the macaques vaccinated with the classical subunit vaccine, those immunized with the iscom vaccine developed high titres of specific IgM, IgA and IgG serum antibodies, as well as high titres of haemagglutination-inhibiting and virus-neutralizing serum antibodies. Also, specific proliferative T cell responses were only found in the iscom-vaccinated monkeys and their levels were similar to those found in monkeys experimentally infected with the homologous virus. Upon intratracheal challenge with the homologous virus, the iscom-vaccinated monkeys were completely protected from detectable virus replication in lungs, pharynx and nose, whereas those vaccinated with the classical subunit vaccines were not, or were only partially protected. The kinetics of specific serum antibody development in the iscom-vaccinated monkeys after challenge were quite similar to those of monkeys after secondary infection with the same virus. In contrast, the post-challenge kinetics of serum antibody development in the monkeys vaccinated with the classical subunit vaccines resembled those of naive monkeys, confirming that these vaccines only provided limited protection in such animals.

[Influenza in the 1995/'96 season; vaccine composition for the 1996/'97 season]

The 1995/'96 season in the Netherlands was marked by an influenza A/H3N2 epidemic that peaked in week 5I. In this week, 39 patients with influenza-like illness per 10,000 inhabitants contacted the sentinel physicians. With two exceptions, influenza A/H3N2 viruses exclusively were isolated during this epidemic period. In the first few months of 1996, a substantial number of influenza A/H1N1 and influenza B viruses were isolated as well. Serological characterization of the circulating viruses revealed that they all resembled the virus strains of the influenza vaccine of 1995/'96, which therefore probably will have provided good protection. Based on the epidemiological data from other countries and the fact that similar H3N2 viruses have been circulating since 1993, the World Health Organization has recommended to exchange the H3N2 component of the 1996/'97 vaccine for a Wuhan/353/95 (H3N2)-like strain.

[Influenza in the 1994/95 season; composition of vaccine for the 1995/96 season]

The 1994/'95 season in the Netherlands was marked by a limited influenza activity which only emerged in late February. The influenza activity remained elevated until the end of April, which is unusually late, and epidemic activity was only reported in the south of the country. Both influenza A/H3N2 and B viruses were isolated in this period. In addition, influenza A/HINI viruses were isolated for the first time since March 1993, from two patients. The majority of the influenza A strains that circulated in the Netherlands in 1994/'95 reacted well with ferret antiserum raised against the strains of the 1994/'95 influenza vaccine, which therefore probably offered good protection. The reactivity of the B strains to antiserum raised against the vaccine strain, B/Panama/45/90, was only moderate, which implies that the protection against the Dutch influenza B strains was not optimal. Based on the results of the worldwide influenza surveillance, the World Health Organization (WHO) has recommended an alteration in both the A/H3N2 and the B component for the vaccine of 1995/1996.

Antigenicity and immunogenicity of recombinant envelope glycoproteins of SIVmac32H with different in vivo passage histories

Shortly after infection of two rhesus monkeys (Macaca mulatta) either with a SIVmac32H challenge stock or with the same virus that had been passaged in another rhesus monkey for 11 months, SIV-envelope genes were cloned from their peripheral blood mononuclear cells and subsequently expressed by recombinant vaccinia viruses. The molecular weights and antigenicities of the thus produced envelope glycoproteins were largely identical to those of the native SIV. The envelope glycoprotein derived from the in vivo passaged virus proved to be poorly recognized by virus neutralizing monoclonal antibodies directed against one of the seven antigenic sites for which monoclonal antibodies were available. Immunization studies in rats showed that this protein was also less efficient in inducing antibodies against this antigenic site, and that it induced significantly lower levels of virus neutralizing antibodies than the other SIV-envelope glycoprotein. The immunogenicity of the SIV-envelope glycoprotein incorporated into immune stimulating complexes (iscoms) was compared to that of the same protein presented with Quil A or MDP-tsl.

Neutralization of feline immunodeficiency virus by polyclonal feline antibody: simultaneous involvement of hypervariable regions 4 and 5 of the surface glycoprotein

Sites involved in antibody-mediated neutralization of feline immunodeficiency virus were mapped by reciprocal exchange of envelope fragments or amino acids between molecular clones of feline immunodeficiency virus with different susceptibilities to neutralization by a polyclonal cat serum. Combinations of mutations within HV-4 or within HV-4 and HV-5 changed the susceptibility of the viruses to neutralizing antibody.

Enhancement of feline immunodeficiency virus infection after immunization with envelope glycoprotein subunit vaccines

Cats were immunized three times with different recombinant feline immunodeficiency virus (FIV) candidate vaccines. Recombinant vaccinia virus (rVV)-expressed envelope glycoprotein with (vGR657) or without (vGR657 x 15) the cleavage site and an FIV envelope bacterial fusion protein (beta-Galactosidase-Env) were incorporated into immune-stimulating complexes or adjuvanted with Quil A. Although all immunized cats developed antibodies against the envelope protein, only the cats vaccinated with the rVV-expressed envelope glycoproteins developed antibodies which neutralized FIV infection of Crandell feline kidney cells. These antibodies failed to neutralize infection of thymocytes with a molecularly cloned homologous FIV. After the third immunization the cats were challenged with homologous FIV. Two weeks after challenge the cell-associated viral load proved to be significantly higher in the cats immunized with vGR657 and vGR657 x 15 than in the other cats. The cats immunized with vGR657 and vGR657 x 15 also developed antibodies against the Gag proteins more rapidly than the cats immunized with beta-Galactosidase-Env or the control cats. This suggested that immunization with rVV-expressed glycoprotein of FIV results in enhanced infectivity of FIV. It was shown that the observed enhancement could be transferred to naive cats with plasma collected at the day of challenge.

A determinant of feline immunodeficiency virus involved in Crandell feline kidney cell tropism

Viral progeny of the molecular clone 19k1 of feline immunodeficiency virus (FIV) can infect feline T-cells but not Crandell feline kidney (CrFK) cells. In contrast, the biological isolate FIV-AM6c, which was CrFK adapted by co-cultivation of FIV-AM6 infected thymocytes with CrFK cells, can infect both thymocytes and CrFK cells. The envelope gene of FIV-AM6c was amplified by polymerase chain reaction using DNA from infected CrFK cells, and subsequently cloned and sequenced. To map viral determinants of CrFK cell tropism, chimeric viruses with a 19k1 background containing envelope gene fragments of FIV-AM6c were constructed. CrFK cells were transfected with DNA of these chimeric clones and co-cultivated with thymocytes. After 3 days the CrFK cells and the thymocytes were cultured separately. FIV antigen could be detected in most of the thymocyte cultures within 14 days and in one of the CrFK cultures after 52 days. The resulting virus from this CrFK culture can infect both CrFK cells and thymocytes. The results of this study indicate that the envelope region contains determinants of CrFK tropism. The delay in replication indicates that also determinants other than those identified here are involved in CrFK cell tropism. More chimeric clones are being studied at present to map these determinants.

Two different mutations in the envelope protein of feline immunodeficiency virus allow the virus to escape from neutralization by feline serum antibodies

Viral progeny of two molecular clones of feline immunodeficiency virus (FIV), 19k1 and 19k32, were tested in a virus neutralization assay. In this assay the infection of thymocytes with FIV19k1 was neutralized by serum S1422, derived from an SPF cat 22 weeks after infection with FIV19k1. We previously reported that a point mutation at position 560 in hypervariable region-5 (HV-5) of 19k1 confers resistance to virus neutralization (Siebelink et al., 1993, J. Virol. 67:2202-2208). Viral progeny of the other molecular clone, FIV19k32, which differs in the envelope protein in only six amino acids from 19k1, was not neutralized. In order to map sites involved in virus neutralization we constructed chimeric clones by reciprocal exchange of 19k1 and 19k32 envelope gene fragments. Reciprocal exchange of a 1662 bp fragment, encoding almost the whole surface protein, which differs in five amino acids between these two clones, resulted in exchange of the phenotype. Amino acids of the envelope protein of 19k1 and 19k32, in which these clones differ, were substituted by point mutation. We demonstrated that one of these mutations, a substitution of leucine to serine at position 483 in HV-4, also conferred resistance of 19k1 to neutralization by serum S1422.

Removal of the cleavage site of recombinant feline immunodeficiency virus envelope protein facilitates incorporation of the surface glycoprotein in immune-stimulating complexes

Recombinant vaccinia viruses were constructed that expressed the complete env gene of feline immunodeficiency virus with or without the nucleotide sequence encoding the cleavage site between the surface (SU) protein and the transmembrane (TM) protein. The removal of this cleavage site resulted in the expression of a 150K protein that was processed into a 130K protein and was not cleaved into the SU and the TM proteins. Removal of the cleavage site also facilitated incorporation of the SU protein in immune-stimulating complexes (iscoms). Antibody responses to both an SU and a TM peptide representing two immunodominant B cell epitopes were measured. These were higher in cats immunized with iscoms prepared from the cleavage site-deleted envelope protein than in cats immunized with iscoms prepared from the native envelope protein or immunized with the envelope protein and the adjuvant Quil A.

gag- and env-specific serum antibodies in cats after natural and experimental infection with feline immunodeficiency virus

In order to monitor the antibody response to feline immunodeficiency virus (FIV) in cats, following experimental and natural infection, enzyme-linked immunosorbent assays (ELISAs) were developed using recombinant env and gag proteins and p24-specific monoclonal antibodies. It was shown that in experimentally infected cats an env protein-specific antibody response was directly followed by a gag protein-specific response. Furthermore, an ELISA for the detection of env protein-specific serum antibodies proved more sensitive in identifying experimentally and naturally infected cats than ELISAs demonstrating gag protein-specific antibodies. It was concluded that, like in HIV infection of humans, the detection of env protein-specific serum antibodies in addition to gag protein-specific antibodies is not only an important tool in the diagnosis of the infection but also in studies concerning the pathogenesis of the disease.

A single amino acid substitution in hypervariable region 5 of the envelope protein of feline immunodeficiency virus allows escape from virus neutralization

We infected a specific-pathogen-free cat (cat 14) with molecularly cloned feline immunodeficiency virus clone 19k1 (FIV19k1 [K. H. J. Siebelink, I. Chu, G. F. Rimmelzwaan, K. Weijer, A. D. M. E. Osterhaus, and M. L. Bosch, J. Virol. 66:1091-1097, 1992]). Serum of this cat obtained 22 weeks postinfection (serum 1422) neutralized FIV19k1 but not FIV19k32, which is 99.3% identical to FIV19k1 in the envelope gene. Serum 1422 also neutralized virus isolated from cat 14 at weeks 2 and 32 postinfection. We then cultured FIV19k1 in the continuous presence of serum 1422, which resulted in a delay in virus replication of 6 weeks. The resulting virus population appeared to be resistant to virus neutralization by serum 1422. Nucleotide sequencing of the env open reading frame of this presumed escape mutant revealed the presence of one silent and two substitution mutations, both of the latter in hypervariable region 5. Through the construction of chimeric viruses and site-directed mutagenesis, we demonstrated that one of these mutations, the substitution of lysine to glutamine at amino acid position 560 in hypervariable region 5, was sufficient to allow the escape of FIV19k1 from neutralization by serum 1422.

Isolation and partial characterization of infectious molecular clones of feline immunodeficiency virus obtained directly from bone marrow DNA of a naturally infected cat

Replication-competent molecular clones of feline immunodeficiency virus (FIV) were isolated directly from the DNA of bone marrow cells of a naturally FIV-infected cat. After transfection in a feline kidney cell line (CrFK) and subsequent cocultivation with peripheral blood mononuclear cells (PBMC), the viral progeny of the clones was infectious for PBMC but not for CrFK cells. PBMC infected with these clones showed syncytium formation, a decrease in cell viability, and gradual loss of CD4+ cells. The restriction maps of these clones differed from those obtained for previously described molecular clones of FIV derived from cats in the United States. The predicted amino acid sequence similarity of the envelope genes of the two clones was 99.3%, whereas the similarities of the sequences of the clones to those of two molecular clones from the United States, Petaluma and PPR, were 86 and 88%, respectively. Most of the differences between the amino acid sequences of the two clones and those of the clones from the United States were found in five different hypervariable (HV) regions, HV-1 through HV-5. The viral progeny of one of these clones was inoculated into two specific-pathogen-free cats. The animals seroconverted, and the virus could be reisolated from their PBMC.

Induction and characterization of monoclonal anti-idiotypic antibodies reactive with idiotopes of canine parvovirus neutralizing monoclonal antibodies

Monoclonal anti-idiotypic (anti-Id) antibodies (Ab2) were generated against idiotypes (Id) of canine parvovirus (CPV) specific monoclonal antibodies (MoAbs). The binding of most of these anti-Id antibodies to their corresponding Id could be inhibited by antigen, thus classifying these anti-Id antibodies as Ab2 gamma or Ab2 beta. By inhibiting experiments it was shown that these anti-Id antibodies did not recognize interspecies cross-reactive idiotopes, but recognized private idiotopes, uniquely associated with the Id of the anti-CPV MoAb used for immunization. This classifies these anti-Id antibodies as non-internal image Ab2 gamma. The potential use of these non-internal image anti-Id antibodies for the induction of antiviral antibodies in the CPV system is discussed.

The use of enzyme-linked immunosorbent assay systems for serology and antigen detection in parvovirus, coronavirus and rotavirus infections in dogs in The Netherlands

Complex trapping blocking (CTB) enzyme-linked immunosorbent assays (ELISAs) and indirect ELISAs for the detection of antibodies to canine parvovirus (CPV), canine coronavirus (CCV) and rotavirus in sera of dogs were established. Double antibody sandwich ELISAs for the detection of CPV-, CCV- and rotavirus antigens in fecal samples were also developed. Both the serological and antigen-detection ELISAs were used to screen samples from dogs in The Netherlands, with or without a history of acute diarrhea. It was shown that the results of the respective serological ELISAs correlated well and that CPV was the major cause of virus-induced acute diarrhea in dogs in The Netherlands.

Feline immunodeficiency virus (FIV) infection in the cat as a model for HIV infection in man: FIV-induced impairment of immune function

To assess the value of feline immunodeficiency virus (FIV) infection as a model for human immunodeficiency virus (HIV) infection in man, we studied the impairment of certain immunological functions following natural or experimental FIV infection. Proliferative responses of peripheral blood mononuclear cells (PBMC) from symptomatic and asymptomatic cats after naturally or experimentally acquired FIV infection, induced by activation with the mitogens concanavalin A, pokeweed mitogen, or lipopolysaccharide or by stimulation with human interleukin-2 (IL-2), were significantly lower than the proliferative responses found with PBMC from noninfected control cats. Also IL-2 production levels of mitogen-activated PBMC from naturally infected symptomatic cats were significantly reduced. These data confirm that the pathogenesis of FIV infection in the cat, like HIV infection in man, is characterized by a serious malfunction of the immune system.

A synthetic peptide derived from the amino acid sequence of canine parvovirus structural proteins which defines a B cell epitope and elicits antiviral antibody in BALB c mice

Synthetic peptides, recombinant fusion proteins and mouse monoclonal antibodies were used to delineate a B cell epitope of the VP'2 structural protein of canine parvovirus (CPV). Although this epitope is not preferentially recognized in the normal antibody response to CPV, virus-specific antibodies could be induced in BALB/c mice with a synthetic peptide representing the epitope. The potential of this non-dominant B cell epitope to induce antiviral immunity in the presence of maternal CPV-specific antibodies, is discussed.

Delineation of canine parvovirus T cell epitopes with peripheral blood mononuclear cells and T cell clones from immunized dogs

Three synthetic peptides derived from the amino acid sequence of VP2 of canine parvovirus (CPV) which were recently shown to represent three distinct T cell epitopes for BALB/c mice could prime BALB/c mice for a CPV-specific proliferative T cell response upon immunization. Proliferative responses of peripheral blood mononuclear cells (PBMC) from CPV-immunized dogs upon stimulation with these and other peptides, covering the major part of the sequence of VP2', identified the presence of T cell epitopes for this species. Most of these epitopes were recognized by PBMC from only a minority of the dogs tested. With three newly generated canine Thyl+ T cell clones, which recognized CPV antigen in association with major histocompatibility complex class II molecules, two distinct T cell epitopes were identified within the unique sequence of VP1.

Establishment and characterization of canine parvovirus-specific murine CD4+ T cell clones and their use for the delineation of T cell epitopes

Canine parvovirus (CPV)-specific T cell clones were generated by culturing lymph node cells from CPV-immunized BALB/c mice at limiting dilutions in the presence of CPV antigen and interleukin-2 (IL-2). All isolated T cell clones exhibited the cell surface phenotype Thy1+, CD4+, CD8- and proliferated specifically in response to CPV antigen. After stimulation with CPV antigen in culture the T cell clones produced IL-2 and proliferated in the absence of exogenous IL-2. Naive mice to which CPV-specific T cell clones had been adoptively transferred developed a CPV-specific delayed type hypersensitivity reaction upon simultaneous intracutaneous injection of CPV in their ears. The ability of recombinant viral fusion proteins, representing the VP2 capsid protein of the antigenically closely related feline panleukopenia virus and of synthetic peptides derived from the amino acid sequence of the VP2 of CPV, to stimulate these T cell clones enabled the identification of T cell epitopes.

Evaluation of enzyme-linked immunosorbent assays based on monoclonal antibodies for the serology and antigen detection in canine parvovirus infections

An enzyme-linked immunosorbent assay (ELISA) system was developed for the detection of canine parvovirus (CPV) or CPV antigen in dog faeces and two other ELISA systems were developed for the detection of CPV-specific antibodies in dog sera. The ELISA's were based on the use of CPV-specific mouse monoclonal antibodies, which recognise different epitopes of the haemagglutinin of CPV and which also neutralise the virus. A double antibody sandwich (DAS) ELISA for the detection of CPV in dog faeces was compared with the haemagglutination (HA) test. The DAS-ELISA proved to be more specific, sensitive and easier to perform than the HA assay. An indirect ELISA and a competitive ELISA for the detection of CPV-specific antibodies in dog sera were compared with the haemagglutination inhibition (HI) test. Both ELISA systems proved to be specific and easy-to-use methods for the detection of CPV-specific antibodies. The indirect ELISA, specially, proved to be more sensitive than the HI test. The higher sensitivity and specificity of the ELISA's as compared to HA and HI tests, and their ease of use, make them suitable for routine use in the serology and diagnosis of CPV infections.

Induction of anti-viral immune response by immunization with monoclonal anti-idiotype antibodies directed to private idiotopes

Syngeneic monoclonal anti-idiotypic antibodies were raised against idiotopes on neutralizing monoclonal antibodies with specificity for feline leukemia virus and canine parvovirus. The anti-idiotypic antibodies were shown to recognize paratope-related private idiotopes. Mice were injected with the monoclonal anti-idiotypic antibodies and the sera of these mice were screened for antiviral reactivities. Antibodies to both feline leukemia and canine parvovirus could be induced as determined by ELISA. These results suggest that anti-idiotypic antibodies which detect private idiotopes and thus do not represent internal images of viral antigens may be considered as candidates for the induction of antiviral immunity.

Purification of infectious canine parvovirus from cell culture by affinity chromatography with monoclonal antibodies

Immuno affinity chromatography with virus neutralizing monoclonal antibodies, directed to the haemagglutinating protein of canine parvovirus (CPV) was used to purify and concentrate CPV from infected cell culture. The procedure was monitored by testing the respective fractions in an infectivity titration system, in an ELISA, in a haemagglutination assay and by negative contrast electron microscopy to quantify CPV or CPV antigen. The degree of purification was further estimated by testing the fractions for total protein content in a colorimetric method, for bovine serum albumin content in an ELISA and by SDS-PAGE. Over 99% of the contaminating proteins proved to be removed, and 20% or 70-90% of infectious CPV or CPV antigen, respectively, was recovered.

[Canine parvovirus infection in dogs: a consideration]

The current knowledge of canine parvovirus (CPV) and the clinical symptoms associated with CPV infection within seven years after the first outbreaks of the disease are reviewed in the present paper. The most important symptoms result from the occurrence of acute enteritis and/or acute myocarditis. Besides characteristics of the virus, symptoms of disease and (histo)pathological findings, particular attention is focussed on recent developments in diagnosis and prevention. A protocol for the prevention of CPV infections in situations in kennels, consisting of combined vaccination and hygienic procedures is presented.

Histology of the bone marrow antibody response. An immunocytochemical study during the secondary response in mouse and rat

The histology of the specific and non-specific antibody response in mouse and rat bone marrow was studied after subcutaneous priming and intravenous boosting with horseradish peroxidase (HRP). Cells producing specific antibody against HRP were found only occasionally in the bone marrow after subcutaneous priming. After the intravenous boost injection their number gradually increased. These anti-HRP forming cells were found as single cells, randomly dispersed throughout the bone marrow. Such a random distribution was also found for cytoplasmic (non-specific) immunoglobulin containing cells. At no time point after immunization could lymphoid aggregates or trapping of immune complexes be observed in the bone marrow of either species. On the basis of these observations it is concluded that the bone marrow forms a suitable microenvironment for immigrating antibody-forming cells but does not contribute actively to the induction of the immune response.