Antigens are classified into two types according to the nature: complete antigen and incomplete antigen.

Incomplete antigen, also known as hapten, is an immunoreactive, non-immunogenic substance. After the hapten is combined with the protein carrier, immunogenicity is obtained. It can also be divided into complex haptens and simple haptens. The complex hapten is not immunogenic and is only immunoreactive, such as most polysaccharides (such as the capsular polysaccharide of pneumococci) and all lipids; simple hapten is neither immunogenic nor immunoreactive However, it prevents the binding of the antibody to the corresponding antigen or complex hapten.

Complete antigen is referred to as antigen. It is a class of substances that are both immunogenic and immunoreactive. Most proteins, bacteria, viruses, bacterial exotoxins, etc. are complete antigens.

The following is a list of research progress in hepatitis virus:

Hepatitis B Virus
Hepatitis B virus (HBV) infection can cause various types of liver diseases, including acute and chronic hepatitis B, cirrhosis, and even liver cancer. Since the first discovery of hepatitis B surface antigen (HBsAg) in 1967, HBsAg has gradually evolved from a qualitative diagnostic indicator for chronic hepatitis B virus infection to a widely used quantitative diagnosis and detection index. HBsAg is HBV protein and rarely turns negative. HBsAg seroconversion is one of the important indicators of viral clearance and disease rehabilitation. At present, HBsAg detection has been widely used in clinical routine qualitative and quantitative detection to guide the anti-HBV efficacy analysis. The current use of HBsAg is increasingly important in the clinical treatment of anti-HBV.

HBV is a small DNA virus belonging to the family of hepadnaviridae. After HBV enters the cell, the protein capsid is first removed, and a portion of the circular double-stranded DNA of HBV is exposed. After the untwisted DNA enters the nucleus, part of the circular double-stranded DNA is repaired into a complete double-stranded, covalently closed circular DNA (ccc DNA), under the action of DNA polymerase. Subsequently, ccc DNA as a stable, stubborn, persistent non-integrating microchromosome persists in the nucleus of the liver cell and becomes a viral gene transcription template, so that chronic HBV carriers continue to produce HBsAg.

Hepatitis B virus can have three forms of particle structure under an electron microscope: large spherical particles having a diameter of about 42 nm, small spherical particles having a diameter of about 22 nm, and tubular particles. The Dane particle has a double-layered outer shell. The membrane protein of the outer shell is the hepatitis B surface antigen (HBsAg), and the latter is comprised of three carboxyl co terminal HBs proteins termed large (LHBs), middle (MHBs) and small (SHBs, also called major) protein. SHBs are mainly composed of spherical particles of 17 to 25 nm, which are very rich in content, about 10 000 times the number of LHBs with infectious particles. In addition, SHBs are also secreted into the blood as fibrillar (20 nm in diameter) or globular (20 to 22 nm in diameter) sub-viral particles, but these empty subviral particles are not infectious. All three forms of HBs protein (SHBs, MHBs, LHBs) can be used in clinical tests. The inner shell is a core protein called hepatitis B core protein (HBc), and which contains the viral polymerase and viral genome.

The hepatitis B virus genome has four different open reading frames (ORFs) encoding the envelope, core, polymerase and X protein. The four types of antigenic proteins, ie, surface antigens (HBsAg), include SHBs encoded by the S region, MHBs encoded by the PreS/S, LHBs encoded by the PreS1/PreS2/S region, and hepatitis B virus core antigen (HBcAg), hepatitis B virus X antigen (HBxAg) and hepatic e antigen (HBeAg). In addition, the three surface proteins of HBV (SHBs, MHBs and LHBs) are generated by translation of different translation initiation sites in a common open reading frame, so their structures have similar parts, and SHBs are 226 on the endoplasmic reticulum. The amino acid hydrophobins, MHBs and LHBs all contain SHBs structure, but MHBs increased pre-S2 region (55 amino acids) based on SHBs, and LHBs increased pre-S2 and pre-S1 regions based on SHBs (108 or 119 amino acids).

HBsAg has a complex molecular structure and is rich in biological functions, but is susceptible to mutation. The clinical value of HBsAg quantification has received extensive attention, especially as a predictor of antiviral response. And the most significant is that HBsAg may be used as a reference indicator for the end point of hepatitis B antiviral therapy in the future. The current US Liver Disease Annual Meeting and the European Liver Disease Annual Meeting Guidelines on the endpoint of anti-HBV treatment are still unclear. Regular monitoring of HBsAg quantification can be used in patients with HBV DNA below the lower limit of detection to determine the endpoint of anti-HBV therapy and the effectiveness of antiviral therapy. Quantitative monitoring of HBsAg levels will help to develop the best treatment and management strategies for CHB patients.

Hepatitis C virus
HCV is an enveloped virus with a diameter of 30-60 nm. The genome is a positive strand RN A with a full length of 9401 nucleotides and can encode 3010 or 3011 amino acids. Its structure resembles flavivirus and prion, and is 5-terminal non- the coding region (5-NC), the middle coding region and the 3-terminal non-coding region (3-NC). The coding region accounts for 95% of the whole genome length, and encodes structural proteins (core protein (C), envelope protein (E1, E2/N S1) and non-structural proteins, namely NS2, NS3, NS4 and NS5 proteins. Structural proteins are involved in the assembly of viral particles, while non-structural proteins are involved in the replication of viral genomes. The 5-NC region is the most conservative, and the 3-NC region has the largest variation. The exact function of the two regions is still unclear. The conservation of the encoded protein is diminished from C, HCV NS3 protein, NS5, NS4, E2 / NS1, to E1. Another distinguishing feature of the HCV genome is variability, which has been recognized as having "missing mutations" and "gene drifts".

The HCV core region is a target gene region for HCV gene and serotyping, and is also an important region for the treatment of HCV infection and vaccine research at the genetic level. Therefore, in recent years, the HCV core region and its gene product core protein (C protein) have been become a hot spot in the current research on the pathogenesis and prevention of HCV.

The C protein is a non-glycosylated protein produced by the cleavage of the HCV polyprotein precursor by a cellular signal peptidase, consisting of 191 amino acids, which can be phosphorylated by protein kinases and present in infected cytoplasm and/or nucleus. The amino acid sequence is very conservative. In the anti-HCV protective vaccine study, it was found that the amino acid sequence of the c protein has multiple epitopes of B, CTL and TH cells. It can induce cross-immune responses of different virus strains. There are several linear B cell epitopes at the N-terminus of the C protein, and the corresponding antibodies can be detected in the patient's serum. And the core region antibody appeared earlier (about 8-10w after infection), lasted for a long time, consistent with abnormal serum transaminase levels, and correlated with HCV RNA titer. Some scholars used the PC (341.910) recombinant plasmid DNA encoding the c protein gene to immunize Balb/c mice, and high titers of anti-C region antibodies appeared in the serum of mice, suggesting that the C-region prototype DNA vaccine can be high in vivo. The c protein is expressed horizontally and elicits a humoral immune response. Although anti-HCV antibodies can persist for a long time in most HCV-infected individuals, HCV infection is mainly characterized by chronic persistence, indicating that antibodies in the host are effectively immune to HCV deficiency, so HCV-infected organisms rely more on cellular immune responses especially CTL and TH.

Peripheral blood mononuclear cells in patients with chronic hepatitis C showed CTL responses to MHC class I molecules in the HCV structural regions and non-structural regions. In vaccine development, endogenous synthetic antigens with epitopes from conserved regions of HCV and treated intracellularly are better viable vaccines. Based on this strategy, multiple CTL epitopes associated with HLA were identified in the C protein amino acid sequence. Protein C 28-37 and HLA-B60.111-130 and 161-180 with DRBI, 88-96 and HLA-B. These short peptides are effective in stimulating CTL recognition of endogenous C proteins, especially the amino acid sequence of amino acids 88-96. If the patient does not show a CTL response to the short peptide, there will be no other CTL epitope response. And when the epitope CTL is activated, the serum HCV RNA titer decreases.

CTL is the main cell that controls HCV infection. CTL epitopes that recognize protein C are often identified by in vitro stimulation of synthetic peptides. The CTLs formed by the polypeptides undergo a process of treating the viral antigens by the cells, that is, the CTL precursors mature and differentiate into CTLs with killing function, and need to recognize the epitopes bound to the MHC class I molecules on the surface of the APC. In the cellular immune response, TH cells recognize the MHC class II molecule and the virus-derived peptide complex on the surface of the cell infection mainly through the II cell receptor, secretion of a variety of cytokines, play an immunomodulatory role. In the presence of anti-viral TH cell epitopes in protein C, it is believed that almost all HCV-derived proteins can stimulate TH cell responses, mainly to produce a response to protein C. and patients who often clear the virus respond more strongly than patients with chronic HCV. The presence of the TH epitope can not only enhance the HCV-specific immune response, but also enhance the antibody's immunity against HCV infection by increasing the antibody level by the intrinsic action with the B cell epitope.

…to be continued in part three.

Reference

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[4] Poordad F, Jr M C J, et al. Boceprevir for untreated chronic HCV genotype 1 infection. [J]. N Engl J Med, 2011, 364(13):1195-1206.