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The H5N1 AIVs continuously outbreak in East Asia countries after 2005 and form a new
phylogenetic clade, gaining some new characterizations, such as neurovirulence and the
ability to infect poultry and migratory birds [7,9]. Meanwhile, most of the subsequent viruses
isolated from the infected human in China prove to be clade 2.3 . H5N1 HPAIVs cause a
constant increase of human infection cases and nearly 60% of the infected people die
Meanwhile, H5N1 HPAIVs are raging all over the world in poultry as well as
migrated birds and putting significant threat to human health and poultry breading.
Further analysis reveal that the HA gene of these viruses belong to Clade 2.3. The properties
of the sequences, virulence and even the transmission property of the viruses in this clade are
investigated profoundly in many reports [7,11-15]. Wu et.al show that viruses isolated from
2002 to 2007 have formed four distinct antigenic groups, and imply that a major antigenic
drift might happen in the viruses isolated after 2005 (clade 2.3) . But the antigenicity of
the viruses in this clade is not fully studied. Investigation about the antigenicity of H5N1
AIVs isolated in different time and geographical areas after 2005 is beneficial to clarifying
the changed biological properties of the emerged viruses.
In the present study, combining the HI and the NT activity of the Mabs to the 10 selected
viruses isolated in central China, we conclude that a significant antigenic drift have occurred
in the viruses isolated in the two periods. To reveal the molecular mechanism of the antigenic
change, phylogenic analysis is performed and displays that HA genes of the viruses isolated
in two periods obviously locate in different branches. The deduced amino acids demonstrate
that the 53 substitutions in the HA protein of the 10 viruses are divided into four patterns. The
pattern 1 mutations are only observed in virus xfy/04, the same sites in other 9 viruses are
identical. Our previous study had demonstrated that the virus xfy/04 showed low
pathogenicity in mice, chickens and pigeons, while the other 9 viruses showed high
pathogenicity in animal experiments or in field [7,17]. Therefore, pattern 1 mutations might
be related to the low pathogenicity of virus xfy/04.
The amino acids in pattern 3 mutations are different in viruses isolated in 2004 and 2006–
2007, indicating that this pattern mutation might happen from 2005 to 2006. However, the
mutations in pattern 2 are only observed in the viruses isolated in 2007, indicating that this
pattern mutation might happen from 2006 to 2007. This information reveals that the
antigenicity of H5N1 AIV is constantly changing in the process of spread.
On the other hand, viruses isolated in 2006 (hm/06 and d16/06) belong to clade 2.3.4, while viruses isolated in
2007 (xn/07, zg/07 and xg/07) belong to clade 2.3.2. Meanwhile, viruses in clade 2.3.2 have
gradually replaced the viruses in clade 2.3.4 in these years and become the most prevalent
viruses in China. This might suggest that viruses undergoing antigenicity alteration have a
great advantage in prevalence and spread.
The present study reveals that the antigenic epitopes of the HA protein have changed in the
prevalent AIVs in central China. The mutations in pattern 2 and 3 might be responsible for
the differences of HI and NT activity and the distinct antigenic features of the viruses isolated
in the two periods. Particularly, most mutations in pattern 2 and 3 locate in the globular head
and distribute at the five epitope sites, and even represent the trend of antigenic drift in 2004–
2007. This phenomenon might demonstrate that those sites actually expose to the selected
pressure and easily cause antigenic drift. Generally, the H5N1 AIVs are undergoing further
antigenic drift and persistent monitoring the epidemiology and antigenic evolution of H5N1
AIV is imperative.