Swine Serologic Cross-Reactivity With Pandemic (H1N1) 2009 Influenza Virus In Parts Of China

The pandemic (H1N1) 2009 influenza virus is a new triple swine-origin reassortant that has caused globally human to human transmission and developed into the first influenza pandemic of the twenty-first century. This specific reassortant had never been found in swine when it was first detected in humans. However, after the outbreak of pandemic influenza H1N1 2009 in human, the pandemic (H1N1) 2009 virus was isolated from swine in more than ten countries during May–November 2009. Moreover, studies had confirmed that pandemic (H1N1) 2009 virus could infect pigs and caused swine to swine transmission. As domestic pigs have been described as a hypothetical'mixing-vessel', mediating by reassortment the emergence of new influenza viruses with avian or avian-like genes into the human population, and triggering a pandemic associated with antigenic shift. Meanwhile, to this day, there is still no evidence of isolation of the pandemic (H1N1) 2009 virus from swine in the mainland of China and the immunity of Chinese domestic pigs to pandemic (H1N1) 2009 virus is unknown. So it is of vital importance to investigate the cross-immunity level of swine in China to this new influenza virus.In order to investigate whether field swine serums in China have cross-reactive antibodies against pandemic (H1N1) 2009 influenza virus, first we respectively amplified the HA1 gene of A/California/04/2009H1N1 (2009 pandemic H1N1 virus)and A/Swine/Shandong/LY/2008(H1N1) (current epidemic SIV H1N1) by PCR, and cloned the PCR products into plasmid pET-30a(+).The recombinant plasmid pET-HA1(2009 pandemic H1N1) and pET-HA1(current epidemic SIV H1N1) were both expressed in in E. coli BL21 strain by the induction of IPTG and the recombinant His-HA1 protein was purified by an Ni-affinity chromatography. Then we respectively developed two enzyme-linked immunosorbent assay (ELISA) to detect antibodies against pandemic (H1N1) 2009 influenza virus and Chinese current epidemic swine influenza A virus (H1N1), using recombinant His-HA1 protein as antigen. Seven hundred and eighty-five field swine serums and one hundred and forty-three human serum obtained from patients diagnosed with pandemic (H1N1) 2009 were collected and detected by these two ELISAs.Then we made correlation analysis between two ELISA results by software Statistical Product and Service Solutions (SPSS), which indicated that there was a significant linear positive correlation between detecting results of SIV (H1N1) HA1-ELISA and that of 2009H1N1 HA1-ELISA. According to this relationship, it could be considered that swine serum positive to SIV (H1N1) had cross-reactivity with 2009H1N1 while human serum obtained from obtained from patients diagnosed with pandemic (H1N1) 2009 had cross-reactivity with SIV (H1N1). Based on this it could be suggested pigs in China had partial immunity to pandemic (H1N1) 2009 virus and in swine serum, the antibody level against pandemic (H1N1) 2009 had direct proportion with that against current epidemic swine influenza virus (H1N1).Combined with the results of sequence alignments, it can be suggested that A/California/04/2009 H1N1 and A/Swine/Shandong/1123/2008(H1N1) may had cross-reactive epitopes, which was the cause of cross-immunity.In this study, we carried out investigated the swine serologic cross-reactivity with pandemic (H1N1) 2009 influenza virus in China. These results enriched the content of pandemic (H1N1) 2009 influenza virus epidemiology in China, and indicated we must enhance the need for systematic surveillance of SIV in China.