Genetic relationship between the HA genes of type A influenza viruses isolated in off-seasons and later epidemic seasons

Generation and properties of one strain of H3N2 influenza virus with enhanced replication

H3N2 canine influenza virus (CIV) has been circulating in many countries since 2008. The epidemic spread of CIV could be a concern for public health because of the close contact between humans and companion animals. In this study, we used Madin-Darby canine kidney (MDCK) cells as a coinfection model of H3N2 CIV and the pandemic (2009) H1N1 influenza virus to investigate the possibility of genetic mutation or recombination. One of the resultant progeny viruses, designated as CP15, was identified with a significantly increased replication ability. For this viral strain all segments exhibit a homology close to 100 % with its parental strain A/Canine/Jiangsu/06/2010 (JS/10), except for two site mutations K156E and R201 K which occur in the receptor-binding sites of hemagglutinin (HA) and antigen binding sites of neuraminidase (NA), respectively. Virus growth in MDCK cells showed that CP15 had a higher virus titer (more than 10 times) than JS/10. Consistent with this, CP15 exhibited extensive tissue tropism and higher viral RNA loads in the spleen, kidney and lung of mice challenged with this virus compared to JS/10. However, body weight loss and lung injure score due to CP15 infection were greatly reduced. Importantly, anti-CP15 serum antibodies could confer a high neutralization activity against JS/10. These findings indicated that the CP15 strain of high replication ability represents a promising candidate to develop an efficient CIV vaccine.

Viral carrier status is instilled by viral regulatory particles

Human viral carriers are important agents in the periodic resurgence of many pathogens. Instillation of virus in human carriers explains several of the unusual epidemiological features of viral epidemics, such as where viruses linger between epidemics and how epidemics can arise without an apparent source. By inactivating itself, a virus can easily reside in a host for months or years without being noticed by the immune system, enabling the virus to be dispersed inconspicuously in the future and into new regions. When this silent activity of human carriers is appreciated, it is easier to understand the dynamics of viral epidemics, such as the explosive appearance of influenza epidemics. During viral illnesses, virus in infected cells is put into a latent state by regulatory sequences delivered by particles produced by other virus-infected cells. These regulatory particles are similar to the virus's virion but contain specific subsets of the viral genome and cannot replicate in cells that are not infected by the complete viral genome. Regulatory particles have previously been referred to as defective interfering particles, noninfective viruses, inactive viruses, incomplete viruses, satellite viruses, and defective viruses. There are still many unanswered questions regarding viral carrier creation and the role human carriers play in the pathology and epidemiology of viral diseases. Some of these questions are presented and discussed in relation to regulatory particles, possible investigations and how carrier status may affect the health of the carrier. Viral regulatory particles limit the extent of viral infections and shift the active infection to a latent infection. Just as multicellular creatures use hormones as chemical messengers to coordinate cellular functions, viruses utilize regulatory particles to coordinate viral modes among infected cells within a host. Many viruses depend on these particles for their continued existence. If we wish to comprehend and effectively treat viral infections, we must secure a thorough understanding of viral regulatory particles.

Restriction of amino acid change in influenza A virus H3HA: comparison of amino acid changes observed in nature and in vitro

We introduced 248 single-point amino acid changes into hemagglutinin (HA) protein of the A/Aichi/2/68 (H3N2) strain by a PCR random mutation method. These changes were classified as positive or negative according to their effect on hemadsorption activity. We observed following results. (i) The percentage of surviving amino acid changes on the HA1 domain that did not abrogate hemadsorption activity was calculated to be ca. 44%. In nature, it is estimated to be ca. 39.6%. This difference in surviving amino acid changes on the HA protein between natural isolates and in vitro mutants might be due to the immune pressure against the former. (ii) A total of 26 amino acid changes in the in vitro mutants matched those at which mainstream amino acid changes had occurred in the H3HA1 polypeptide from 1968 to 2000. Of these, 25 were positive. We suggest that the majority of amino acid changes on the HA protein during evolution might be restricted to those that were positive on the HA of A/Aichi/2/68. (iii) We constructed two-point amino acid changes on the HA protein by using positive mutants. These two-point amino acid changes with a random combination did not inhibit hemadsorption activity. It is possible that an accumulation of amino acid change might occur without order. (iv) From the analysis of amino acids participating in mainstream amino acid change, each antigenic site could be further divided into smaller sites. The amino acid substitutions in the gaps between these smaller sites resulted in mostly hemadsorption-negative changes. These gap positions may play an important role in maintaining the function of the HA protein, and therefore amino acid changes are restricted at these locations.

Analysis of influenza A virus reinfection in children in Japan during 1983-91

The epidemiology of influenza A in Japan was studied during 1979-91 and viruses isolated from reinfections during 1983-91 were analysed. Of 2963 influenza viruses isolated from reinfections during 1983-91 were analysed. Of 2963 influenza viruses isolated during this period, 922 and 1006 were influenza A(H1N1) and A(H3N2) viruses respectively; the others were influenza B viruses. Influenza A(H1N1) and A(H3N2) caused 5 and 6 epidemics respectively, most accompanied by antigenic drift. Seventeen reinfections with H1N1 and 17 with H3N2 were detected during our study. The primary and reinfection strains isolated from 7 H1N1 and 10 H3N2 cases were studied by haemagglutination-inhibition, and amino acid and nucleotide sequences of the HA1 region of the haemagglutinin. Most of the primary and reinfection strains were antigenically and genetically similar to the epidemic viruses circulating at that time. However, in 4 out of 10 cases of reinfection with influenza H3N2 virus, reinfection strains were genetically different from the epidemic viruses.

Genetic and antigenic variation in the haemagglutinin of recently circulating human influenza A (H3N2) viruses in the United Kingdom

Variation in the haemagglutinin (HA) gene of influenza A (H3N2) viruses isolated in the U.K. and abroad from 1992-1994, was determined by nucleotide sequencing of the HA1 domain of the HA gene. Viruses isolated in the U.K. early in the 1992-93 season were from the A/Beijing/353/89 lineage and were replaced later that season by viruses from the A/Beijing/32/92 lineage. Viruses from the new lineage continued to be isolated during the 1993-94 season, but were heterogeneous. Most of these isolates were more closely related to an A/Beijing/32/92 variant, A Hong Kong/23/92, but could be distinguished into three groups by serology (of which one group was circulating during the previous season) and four groups based on sequence variation in the HA gene. However, phylogenetic analysis of antigenically-distinct isolates showed that the HA gene is evolving along one lineage. Sequence analysis identified mainstream, subgroup and strain specific amino acid substitutions. There was a broad correlation between the observed amino acid changes and the antigenic sites of the HA. The results of this study highlight the value of regular molecular analysis of circulating viruses.

Host-specific hemagglutination of influenza A (H1N1) virus

H1N1 strains of influenza A virus isolated during the influenza season of 1991-92 were divided into two groups according to the property of host-specific hemagglutination. Group 1 viruses agglutinated human and chicken red blood cells. Group 2 viruses agglutinated human but not chicken red blood cells. The viruses of both groups, however, showed the same antigenic structure determined with ferret antisera. The virus clones which were plaque-purified twice from a group 2 virus retained the characteristic of host-specific hemagglutination after five successive passages in MDCK cells, indicating that this phenomenon is genetically determined. However, the amino acid sequences of the hemagglutinin (HA) polypeptides deduced from the nucleotide sequences of the HA gene of the two groups did not show any differences between them. This suggests a difference in amino acids in some other polypeptide(s), which affects the host-specific hemagglutination.

Vaccination-induced HI antibody to influenza A(H1N1) viruses in poorly primed adults under circumstances of low antigenic drift

In Autumn 1990, trivalent split influenza virus vaccine containing A/Taiwan/1/86(H1N1) was used to immunize healthy female employees (n = 104). The 11-12 amino acid differences in the HA1 domain of virus haemagglutinin between A/Taiwan/1/86 and representative epidemic H1N1 strains in Finland in 1991 did not result in lowered haemagglutination-inhibiting (HI) antibody responses to the latter viruses. In fact, higher prevaccination, postvaccination and postepidemic antibody titres were recorded against the new field strains than against the vaccine virus; the highest being against field strains grown exclusively in MDCK cell cultures. This pattern is primarily explained by differences in the sensitivity of the viruses for detecting HI antibodies. Postvaccination protection rates of 98-100% for the MDCK-grown avid viruses were noted in subjects who exhibited prevaccination antibody. Lower protection rates were recorded in initially seronegative subjects, the lowest (54-57%) being among older people, i.e. among vaccines born in 1930-1955 (p < 0.001). Moreover, conspicuous decreases in protection rates were detected during the following epidemic season in the initially seronegative subjects. Diagnostic findings during outbreaks due to H1N1 subtype viruses also support the impression that many middle-aged people are poorly primed. Thus, vaccination with two doses may be worth considering when such people join the high-risk group and receive influenza vaccine for the first time.

Comparison of the HA genes of type B influenza viruses in herald waves and later epidemic seasons

From January 1985 to May 1991, herald strains of influenza B virus were isolated in 1987 and 1989 in Japan. In both cases, influenza epidemics caused by the same type followed in the next winter season. The HA gene sequences of the influenza B viruses isolated in Japan from 1987-91, which covers two herald waves of influenza B viruses, were analysed and located on the phylogenetic tree for influenza B viruses after the B/Singapore/64 strain. Co-circulation of at least two evolutionary lineages of the HA genes existed for influenza B viruses in Japan during the period of this study. The herald viruses in one wave (1987) were genetically close to the winter isolates and were considered to be the parental viruses for the following influenza season, while in the other wave (1989) winter isolates belonged to another lineage on which one of the herald viruses was located, but they were genetically and antigenically different from the herald viruses.

Evolution of influenza A(H1N1) viruses during a period of low antigenic drift in 1986-91: sequence of the HA1 domain of influenza A/Finland/158/91

This study used the nucleotide sequence coding for the HA1 domain of virus haemagglutinin to show that influenza A/Finland/158/91, which represents the H1N1 subtype viruses prevalent in Finland in 1990/91, was a direct descendant of a virus (A/NN/1605/88) isolated during the 1988/89 epidemic season in Japan. The elevated rate of 7.4 x 10(-3) nucleotide substitutions per site per year is discussed. The new branch of H1N1 subtype viruses is characterized by loss of a glycosylation site, which may affect subsequent antigenic drift.

Detection of influenza viruses in throat swab by using polymerase chain reaction

An assay protocol based on exploiting the polymerase chain reaction (PCR) for the direct detection of influenza virus in throat swab is described. By use of the mixture of H1 and H3 primers, it was possible to determine the subtype of the influenza A viruses simultaneously. No visible band was detected after PCR of influenza B or A (H2N2) viruses with a pair of H1 or H3 primers. The dilution experiment showed that the influenza viruses, as few as 1.3-6 plaque-forming units, were sufficient for detecting the HA gene by PCR. All throat swab samples from which influenza viruses had been isolated by conventional method were also positively detected by PCR method.