| Pocine Streptococcosis is one of the most important diseases that endanger the pig industry which can cause meningitis,sepsis,arthritis and other symptoms in pigs.It is also a zoonotic disease,which greatly threatens public health.At present,vaccination is an important means to prevent this disease.However,because there are numerous serotypes of Streptococcus suis and there is no cross-protection among them,it is hard to develop a universal Streptococcus suis vaccine.The existing inactivated vaccines can only defend homologous serotypes,and the attenuated vaccines screened by traditional methods are in danger of re-virulence.Therefore,it is an urgent need to develop both safe and effective innovative vaccines to prevent various of Streptococcus suis.The mutant strain of streptococcus suisΔcps/ssna-msly(P353L)-SC19,which was constructed by our previous work,is an attenuated strain obtained by knocking out capsular polysaccharide(CPS)and virulence-related factor secretory nuclease A(Ssn A)on the basis of type 2 epidemic strain SC19,and mutating hemolysin(sly)at site 353 to lose its hemolytic activity.Because this strain is not encapsulated by capsules,the potential universal antigens in it are exposed,so it may have cross-protection against different serotypes of Streptococcus suis infection.Therefore,in this study,the virulence of the attenuated strain to cells and mice was first evaluated.Then the C57 mice were immunized with the attenuated vaccine and inactivated vaccine prepared byΔcps/ssna-msly(P353L)-SC19 strain.Finally,the protective effects of the two vaccines on Streptococcus suis type 2 and type 9 were analyzed in vivo and in vitro.The results are as follows:(1)The growth characteristics ofΔcps/ssna-msly(P353L)-SC19 mutant were similar to those of SC19,but its toxicity to A549 cells decreased significantly.Importantly,intraperitoneal injection of even 4×10~9CFU of this mutant strain did not cause any morbidity and death of mice,indicating that its pathogenicity to C57 mice decreased significantly.(2)The attenuated vaccine prepared with the mutant strain induced significant humoral immunity and cellular immunity of C57 mice,and induced antibodies with significant opsonic killing activity.For the challenge experiment,the attenuated vaccine inhibited the inflammatory reaction,tissue damage and the death caused by SC19infection with 100%protection of mice.Meanwhile,it also reduced the inflammatory reaction and tissue damage caused by SS9 infection,particularly,it alleviated the damage of blood-brain barrier and prevented meningitis,and finally protected 50%of mice from death.(3)In parallel,the inactivated vaccine prepared with the mutant strain induced significant humoral immunity and cellular immunity of C57 mice,and induced antibodies with significant opsonic killing activity.For the challenge experiment,the Inactivated vaccine inhibited inflammation,tissue damage and death caused by SC19 infection,with a protection rate of 75%.Besides,it also reduced the inflammatory reaction and tissue damage caused by SS9 infection,with the protection rate of 100%.In summary,this study proved that the virulence ofΔcps/ssna-msly(P353L)-SC19mutant decreased significantly through the toxicity experiment on cells and pathogenicity experiment on mice.Besides,the attenuated vaccine and inactivated vaccine prepared by this strain had good immunogenicity,which could stimulate high-levels of humoral immunity and cellular immunity,and induced antibodies with opsonization activity.Most importantly,it can cross-protect the infection of Streptococcus suis of different serotypes.Therefore,Δcps/ssna-msly(P353L)-SC19 has the potential to be developed into a universal attenuated vaccine or inactivated vaccine,and our research above provides a new idea for the development of universal vaccine against Streptococcus suis. |
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