Nucleic acid vaccines against hepatitis viruses.
Howard CR. Gray L. D'Mello F. Christopher J. Craske J.
Department of Pathology and Infectious Diseases, Royal Veterinary College, London, U.K.
Direct DNA intramuscular or intradermal injection of plasmids containing viral genes under the control of viral promoters is an efficient means of stimulating both class I and class II-mediated antiviral responses. Viral hepatitis B and C are suitable candidates for this approach, particularly as therapeutic immunogens for chronically infected individuals. Several groups have shown that the S gene of HBV is expressed in murine muscle and stimulates a high titre and long-lasting anti-HBs response. Uniquely, CD8+ CTL responses are also induced to HBsAg. No vaccine exists for HCV. Therefore the structural genes (C + E1 + E2) have been cloned as a 2,831 bp fragment from a genotype la isolate into the vector pcDNA3. The resulting plasmid DNA was injected directly into the quadriceps muscle of three-week-old BALB/c mice. Intracellular-expressed E1 and E2 proteins thus represent the complete spectrum of native structural epitopes, including those dependent on glycosylation and protein folding. Mouse antisera were tested for reactivity against conserved sequences using overlapping 7-mer peptides. Two conserved, overlapping epitopes were identified in E2 spanning residues 581-591 and 590-603. This domain represents one of seven major E2 antigenic domains recognized by HCV human antibodies, one of three with antigenic homologies to related flavivirus proteins. Thus antigen is presented with high efficiency following DNA injection and offers the potential of high rates of seroconversion and virus clearance in those predisposed to virus-induced chronic liver disease.
Antigen-independent activation of resting T-cells in the liver of patients with chronic hepatitis.
Chiron/Vaccines, Siena, Italy.
Since the adult liver is an organ without constitutive lymphoid components, any intra-hepatic T cell found in chronic viral hepatitis should have compartmentalised to the liver after infection and inflammation. In liver biopsies from patients with chronic hepatitis C there is a great discrepancy between the percentage of activated T cells and the frequency of antigen-specific T cells. Usually, 40 to 80% of the liver-infiltrating T cells express activation markers, whereas only 0.5%, at best, of these cells are specific for any HCV protein. This finding suggests that there may be antigen independent mechanisms activating bystander T cells. We investigated whether human resting T cells could be activated to proliferate and display effector function in the absence of TcR occupancy. We have found that a combination of IL-2, TNF alpha and IL-6 could activate highly purified naive and memory T cells to proliferate. Under these conditions, resting memory T cells could also display effector function, as assessed by cytokine synthesis and help for IgG production by B cells. This novel antigen-independent pathway of T cell activation may play an important role in vivo in activating effector T cells in the liver and in maintaining the clonal size of peripheral memory T cells in the absence of antigenic stimulation.