Detection of reverse transcriptase activity in association with the non-A, non-B hepatitis agent(s)

The long and winding road leading to the identification of the hepatitis C virus

This review describes work conducted largely in my laboratory at the Chiron Corporation between 1982 and 1989 that led to the identification of the hepatitis C virus (HCV). Key colleagues included Dr. Qui-Lim Choo in my laboratory and Dr. George Kuo also of Chiron as well as my collaborator Dr. Daniel Bradley at the CDC who provided many biological samples from the NANBH chimpanzee model. Numerous molecular approaches were explored including the screening of tens of millions of bacterial cDNA clones derived from these materials. While this early genomics approach resulted in the identification of many host gene activities associated with NANBH, no genes of proven infectious etiology could be identified. A separate avenue of our research led to the molecular characterization of the complete hepatitis delta viral genome but unfortunately, this could not be used as a molecular handle for HCV. Largely following input from Dr. Kuo, I initiated a blind cDNA immunoscreening approach involving the large-scale screening of bacterial proteomic cDNA libraries derived from NANBH-infectious chimpanzee materials (prior to the development of PCR technology) using sera from NANBH patients as a presumptive source of viral antibodies. Eventually, this novel approach to identifying agents of infectious etiology led to the isolation of a single small cDNA clone that was proven to be derived from the HCV genome using various molecular and serological criteria. This discovery has facilitated the development of effective diagnostics, blood screening tests and the elucidation of promising drug and vaccine targets to control this global pathogen.

Discovery of the hepatitis C virus

After nearly 6 years of intensive investigations between 1982 and 1988 in my laboratory at Chiron corporation, in which numerous molecular biological methods were used to investigate the viral aetiology of parenterally transmitted non-A, non-B viral hepatitis (NANBH), a single cDNA clone (5-1-1) was isolated that was shown to be derived from a new flavi-like virus, termed the hepatitis C virus (HCV). After screening hundreds of millions of bacterial cDNA clones derived from different liver and plasma samples obtained from experimentally infected chimpanzees, a single HCV clone was eventually isolated using a novel, blind immunoscreening method in which antibodies derived from a clinically diagnosed NANBH patient were used to identify a cDNA clone encoding an immunodominant epitope within HCV nonstructural protein 4. Its viral origin was demonstrated by its specific hybridization to a large single-stranded RNA molecule of approximately 10,000 nucleotides found only in NANBH-infected samples that shared distant sequence identity with flaviviruses. Further, HCV clone 5-1-1 was shown to be extrachromosomal and to encode an antigen eliciting antibody seroconversion only in NANBH-infected chimpanzees and humans. Subsequent work demonstrated that HCV was the principal cause of parenterally transmitted NANBH around the world, with an estimated 170 million global carriers and that blood screening tests detecting circulating HCV antibodies and viral RNA could effectively eradicate the transmission of transfusion-associated NANBH. Key viral-encoded enzymes essential to its life cycle are now the targets of vigorous, ongoing drug development activities, and the feasibility of successful vaccination strategies has been demonstrated using the valuable chimpanzee model, without which any progress on HCV would not have been possible. My colleagues and coworkers who made essential contributions to the discovery of HCV were George Kuo, who had his own laboratory at Chiron and who provided intellectual and practical input, Dan Bradley of the Centers for Disease Control and Prevention, who provided a large supply of well-characterized chimpanzee samples and knowledge of the NANBH field, and Qui-Lim Choo, in my own laboratory, who provided many years of outstandingly dedicated and precise molecular biology expertise.

[From non-A non-B hepatitis to hepatitis C]

In 1974, the existence of hepatitis serologically distinct from hepatitis A and B was recognized. They were tentatively designated non-A non-B. They accounted for 90% of post-transfusional hepatitis. During 15 years numerous studies failed to identify agent(s) responsible for these hepatitis. In 1989 the virus responsible for parenteral non-A non-B hepatitis was identified and named hepatitis C virus. Shortly after, the virus responsible for enteral non-A, non-B was also discovered (hepatitis E virus). During these 15 years, a 60% reduction of post-transfusional hepatitis was obtained both by the measures instituted to prevent AIDS transmission and by the introduction of surrogate assays (ALT levels and anti-HBc antibody).

Hepatitis C virus: historical perspective and current concepts

Over the past 30 years, hepatitis C has emerged from shadowy enigma to important public health problem. The existence of the etiological agent of this disease was first appreciated two decades ago but significant progress in its understanding had to await its molecular characterization within the past 5 years. The virus is a member of the family Flaviviridae and is the cause of approximately 20% of clinical viral hepatitis in the United States. While the control of the transmission of hepatitis C virus in blood and blood products has been nothing less than spectacular, the control of community-acquired hepatitis C will be a major challenge to the scientific and medical communities.

Modulation by polyamines of DNA-dependent DNA polymerase activity from human serum

1. Spermine, spermidine and putrescine activated DNA-dependent DNA polymerase from human sera by 47-125% at the concentrations of 0.2, 3 and 30 mM, respectively. 2. The polyamines shifted the optimal MgCl2 concentration for the polymerase activity from 10 mM to more physiological 5 mM. 3. Histamine having amino and imino groups at both ends of the molecule also increased the DNA polymerase activity, while cyclopentylamine and n-butylamine showed no effects on the enzyme activity. 4. The stimulatory effect of polyamines on the DNA polymerase activity was more evident with poly(dC)p(dG) used as a template/primer than with poly(dA)p(dT).

Non-A, non-B hepatitis: visualization of virus-like particles from chimpanzee and human sera

Virus-like particles (VLP), morphologically similar to togavirus particles, were detected in the sera of chimpanzees and humans with chronic non-A, non-B (NANB) hepatitis. Particles resembling the virus were also found in the cytoplasm of hepatocytes obtained from NANB hepatitis infected chimpanzees. Furthermore, two chimpanzees inoculated with materials containing the VLP developed NANB hepatitis. These results strongly suggest that the VLP may be closely associated with the agent of NANB hepatitis.

Transfusion transmitted non-A, non-B hepatitis

Non-A, non-B of hepatitis (NANBH) may occur following blood transfusions or administration of blood products. The causative agent(s) is still not identified and the symptoms are usually mild. The only indication of infection may be increased serum alanine transferase levels. The incidence of posttransfusion NANBH has been reported as high as 4-12% in the US (average 7%) while in Sweden it is according to recent studies on the average 2%. An estimated 2-3% of Swedish blood donors are probably carriers of the NANBH agent(s). Of patients acquiring posttransfusion NANBH, 40-60% will develop chronic hepatitis which in 15-20% will progrediate to cirrhosis.

The impact of molecular biology in the diagnosis and management of viral hepatitis

In recent years molecular biology techniques allowed an extraordinary progress in knowledge of hepatitis viruses. New informations have accumulated on the mechanisms of virus replication and on the pathogenesis of liver damage. In the clinical setting, simple noninvasive methods that use the same technological approach provide a useful diagnostic tool for infections of hepatitis B virus (HBV) and hepatitis delta virus (HDV). Using these tests we may define whether a carrier of hepatitis B surface antigen has a liver damage due to HBV or HDV or to other, non-virological causes. Detection of HBV-DNA and HDV-RNA in serum and liver can also be used as a guide for prognosis and therapy.

Delta and non-A, non-B hepatitis viruses

A review is given on the current knowledge of the hepatitis delta virus (HDV), the only hepatotropic non-A and non-B (NANB) virus characterized, although the infection it causes requires infection with hepatitis B virus (HBV). Studies in chimpanzees have provided the most data now available on the putative NANB agents. Histologic and electron microscopic changes occurring in HDV and NANB hepatitis have been shown to be comparable, and some biologic, epidemiologic and clinical features are similar. However, the lack of cross-protection between NANB and HDV hepatitis in cross-challenge transmission experiments and the lack of hybridization between HDV-RNA and nucleic acids from NANB material indicate that HDV and the putative NANB agents are minimally, if at all, related.

Lack of reverse transcriptase activity in serum in sporadic post-transfusional and presumed epidemic or water-borne forms of severe non-A, non-B hepatitis

Reverse transcriptase (RT) activity was not detected in any serum sample taken from 22 patients with mainly severe non-A, non-B hepatitis (NANBH), using two assays selected to cover the range of known human and animal retroviruses. The study included patients with fulminant and sub-acute hepatic failure, which was was attributed to sporadic, post-transfusional, and presumed epidemic or water-borne epidemiological forms of NANBH. Although we cannot exclude the possibility that some of the agents implicated in NANBH are retroviruses, our negative findings suggest that other agents may be involved at least in the severe forms of NANBH.

Spumaviruses isolated from sources containing agents of non-A, non-B (NANB) hepatitis do not cause NANB hepatitis

Serum and liver tissue containing infective non-A, non-B hepatitis virus were shown to contain a retrovirus-like agent that replicated when inoculated into chimpanzee liver cell cultures in vitro. The virus appeared to assemble its core particles in association with tubular structures reminiscent of those characteristically seen in non-A, non-B hepatitis virus-infected chimpanzee liver in vivo, and produced syncytial cytopathic effects in a number of continuous and a primary mammalian liver cells. The agents were neutralized by acute and convalescent sera from human and chimpanzee cases of non-A, non-B hepatitis, as well as by antisera against simian spumavirus type 7, but not type 6. Aluminum chloride failed to abolish viral infectivity. There was no evidence of virus replication or hepatitis in chimpanzees inoculated with a seventh passage of one of the isolates. Thus the data suggest that the isolates are not causally related to non-A, non-B hepatitis, as was previously postulated.

Chronic non-A, non-B hepatitis. A long-term follow-up study in 49 patients

Forty-nine patients with biopsy-verified chronic non-A, non-B hepatitis (NANBH) of both percutaneously transmitted and sporadic types were followed up for up to 20 years (mean, 62 months +/- 44 months). Drug addicts were not included. Twenty-four patients had chronic persistent hepatitis (CPH), and 25 had chronic active hepatitis (CAH) or cirrhosis on the basis of the first biopsy. Patients with CPH were significantly younger than patients with CAH (mean age, 31 and 51 years, respectively; p less than 0.001). Standard laboratory data (means) correlated with histology, but great variations made liver biopsy essential for the diagnosis. Twenty-one patients were rebiopsied, and 24% had more severe lesions. In total, 16 patients (33%) had signs of cirrhosis. The disease seemed to resolve in eight patients (16%), whereas two patients died of it. Some patients with CPH might progress to CAH, and the frequent finding of cirrhosis in CAH implies the possibility of hepatic failure and fatality in chronic NANBH.

Retroviruses and human disease

Over the past 25 years animal retroviruses have been favoured subjects of research by virologists, oncologists, and molecular biologists. Retroviruses have given us reverse transcriptase, oncogenes, and cloning vectors that may one day be exploited for human gene therapy. They have also given us leukaemia and the acquired immune deficiency syndrome (AIDS). Kawasaki disease and tropical spastic paraparesis are thought to be associated with retrovirus infection, and other diseases such as de Quervain's thyroiditis, multiple sclerosis, acquired hypogammaglobulinaemia, and certain forms of non-A, non-B hepatitis have come under passing suspicion of a retroviral aetiology. With AIDS threatening to become pandemic, and a second AIDS virus appearing in West Africa, human retroviruses are under intensive study for new antiviral drugs targeted to their unique mode of replication, and for the development of vaccines.

Fulminant hepatitis in asymptomatic hepatitis B surface antigen carriers in Greece

Eleven male fulminant hepatitis (FH) patients (mean age: 47.7 +/- 16 years) positive for hepatitis B surface antigen (HBsAg) but negative for IgM antibody to hepatitis B core antigen (IgM anti-HBc) were admitted consecutively to the Athens Hospital for Infectious Diseases between May 1981 and November 1983. Because of the absence of IgM anti-HBc, determined by an enzyme immunoassay, these patients were considered to be HBsAg carriers with a superimposed acute hepatitis. Three of the 11 patients received immunosuppressive chemotherapy during the six months before the onset of the acute hepatitis. None of the patients was homosexual or a drug addict. Infection with hepatitis A virus (HAV), hepatitis B virus (HBV), or hepatitis delta virus (HDV) was detected with serologic markers and/or molecular hybridization techniques. Fulminant hepatitis was attributed to spontaneous reactivation of chronic hepatitis B in four patients, chemotherapy-induced reactivation of chronic hepatitis B in three patients, HDV superinfection in one patient and possible superinfection by non-A, non-B agent(s), HDV, or HDV-like agents in three patients. Reactivation of chronic hepatitis B was an important cause of apparent acute hepatitis in heterosexual male HBsAg carriers from an area with a high prevalence of HBV infection.

Viral hepatitis

Developments over the last four years in our understanding of viral hepatitis are analyzed. The molecular structure of hepatitis A has been established, and vaccines for prevention are under development. The recognition of the replicative and integrated stages of hepatitis B infection has allowed more rational approaches to therapy. Vaccines are of proven value. Delta virus infection has assumed an important role world wide as a cause of serious and fulminant liver disease in hepatitis B carriers. The agents for non-A, non-B virus hepatitis have eluded identification. These are important causes of chronic liver disease particularly in recipients of blood transfusion.

Non-A, non-B hepatitis: an update

The definition of non-A, non-B hepatitis (NANB) is improved by further characterization of what it is not (like the delta agent or non-A epidemic hepatitis) rather than by providing convincing evidence of isolation of the agent responsible for blood transfusion- or blood product-related NANB or specific markers thereof. Yet, NANB research is in disquieting movement. Modern biotechnology yielded its blessings to the field. However, monoclonal antibodies and molecular probes will have to be evaluated with the same scrutiny that unmasked so many test systems and viral agents thus far. Recent victims appear to be published reports on NANB being identified as a retroviral agent and NANB virus being propagated in primary cultures of chimpanzee hepatocytes. Yet the application of these powerful new tools, together with the availability of cultured human and chimpanzee hepatocytes for propagation of the agent may improve the chances for substantial progress. Our finding of involvement of lymphocytes in transmission of the disease may add another approach to reach the ultimate goal of characterization of the causative agent and development of diagnostic methods to detect it in patients and biological materials derived from carriers of the disease.

Clinical approach to viral hepatitis in homosexual men

The prevalence of both hepatitis A and hepatitis B is increased in homosexual men. On an annual basis, 5% to 7% of homosexual men will acquire hepatitis A. Risk factors for HAV infection include length of homosexual activity, number of sexual contacts, and oral--anal sexual contact. The HBsAg carrier rate of homosexual men is 5% to 6%, and another 50% have evidence of previous HBV infection with a positive anti-HBs. HBeAg is present in a higher precentage of HBsAg-positive homosexual men (38% to 75%) than in general population carriers (3% to 30%). The annual incidence for HBV infection in homosexual men is 16% to 28%, higher than that for hepatitis A. Transmission of HBV infection in homosexual men is facilitated by a large number of sexual partners, high HBsAg carrier rate, high infectivity of carriers (positive HBeAg), and the specific sexual practices of oral--anal and anal--genital contact with exposure to HBV on open mucosal surfaces. The prevalence of non-A, non-B and delta infection in homosexual men is probably somewhat increased, but the importance of these viruses in the development of hepatitis in this population remains uncertain. Prevention of hepatitis A and B in homosexual men will ultimately be achieved by vaccination of susceptible individuals, which currently is feasible only for hepatitis B. Appropriate use of immune globulins for postexposure prophylaxis and knowledge of specific sexual practices that transmit disease may reduce the incidence of hepatitis A and B.

Hepatitis in clinical practice. 2. Non-A non-B and delta hepatitis

Non-A non-B (NANB) hepatitis has at least two distinct causative agents. NANB hepatitis can lead to chronic liver disease, cirrhosis, and hepatocellular carcinoma. No definitive policy exists to exclude this disease as a cause of posttransfusion hepatitis. Delta agent is epidemiologically linked to hepatitis B virus (HBV) infection, either as a coinfection or as a superinfection in chronic hepatitis B surface antigen carriers. Recognition of hepatitis due to delta agent will probably increase as testing for this agent becomes more available and its presence becomes more generally well known throughout the medical profession.

Seroepidemiological studies on a non-A, non-B hepatitis specific antigen/antibody system (SO-antigen/anti-SO)

The patients and staff members of a haemodialysis unit were examined for their serological responses to SO-antigen, which was isolated from the urine of epidemic type non-A, non-B hepatitis patients at Tohoku University Hospital. To understand how SO-antigen or SO-antigen-related aetiology can be incriminated for the hepatitis found in the haemodialysis unit, the prevalence of SO-antigen/anti-SO system and hepatitis A and B virus-related antibodies was compared in the sera of patients and staff members. Although the SO-antigen was rarely detected in the serum, anti-SO antibody was frequently detected in the sera of patients and staff. A significantly higher prevalence was found in the serum of patients (15%, 54 out of 361) than staff members (7.1%, 13 out of 184) and volunteer blood donors (1%, 3 out of 305). The same prevalence percentages of HBV-related antibodies (either positive for anti-HBs or anti-HBc) and anti-HAV were observed among the patients, staff, and volunteer blood donors, irrespective of whether the sera were anti-SO positive or negative. Among the staff, anti-SO antibody was more frequently found in those with a history of acute hepatitis (16.7%, 3 out of 18) than in those without (6%, 10 out of 166). These prevalence ratios conformed with those of HBV-related antibodies, but the same prevalence ratios of antibody to HAV were observed between the staff with and without a history of acute hepatitis. These results indicate that the SO-antigen/anti-SO system or entity related to this immune system is distinct from HBV or HAV, and this immune system was found widely in the haemodialysis unit where type B and non-A, non-B hepatitis were also found frequently.

Retrospective investigation of transfusion-related non-A, non-B viral hepatitis associated with M2 antigen

A retrospective study of transfusion-related non-A, non-B viral hepatitis was conducted in seven patients. Findings showed that six recipients had been exposed to at least one unit of blood possessing a putative serological marker for non-A, non-B hepatitis, the M2 antigen or anti-M2IgM. The M2 markers were associated with non-A, non-B hepatitis that developed 3-7 weeks after transfusion. It sees increasingly probable that M2 markers may be useful for diagnosis and/or prevention of post-transfusion hepatitis.

A glycoprotein associated with the non-A, non-B hepatitis agent(s): isolation and immunoreactivity

A glycoprotein was isolated and purified to homogeneity from the serum of a patient with chronic non-A, non-B hepatitis. NaDodSO4/PAGE of the glycoprotein revealed a single major band at Mr approximately 77,000. Antibodies to this glycoprotein were shown to possess the following immunoreactivity: (i) they reacted by radioimmunoassay with sera obtained at the time of diagnosis from 17 of 42 patients with non-A, non-B hepatitis and with only 2 of 58 sera from either matched controls or patients with hepatitis A or hepatitis B, (ii) they reacted with sucrose gradient fractions from a proven infectious non-A, non-B hepatitis serum at a peak density of 1.14 g/ml and in the soluble protein fractions on top of the gradient, and (iii) they reacted in ELISA with disrupted human T-cell lymphocytotropic virus type III (HTLV-III), and (iv) they reacted in immunoblots with a protein of Mr 74,000 derived from HTLV-III.

Transmission of non-A, non-B hepatitis by heat-treated factor VIII concentrate

In-vitro and animal studies have shown that viral agents can be removed from or inactivated in clotting factor concentrates by physical or chemical treatment. However, clinical data have as yet not substantiated the results of these studies. 13 haemophilia A patients who had not been treated previously with blood or blood products were given a dry-heated factor VIII concentrate and were tested serologically over the next 12 months. Hepatitis developed in 11 patients (84%) and was invariably of type non-A, non-B. Morbidity was not related to the lot of the therapeutic material or to the number of infusions. The incubation period was either 5 or 8-11 weeks, and only 1 patient had symptoms. Aminotransferase elevation showed both monophasic and biphasic patterns. During the follow-up period signs of the disease disappeared in 10 patients (90%). These findings contrast with the absence of non-A, non-B hepatitis in chimpanzees given the same heated concentrate. Thus, clinical studies in first-exposure haemophiliacs are essential for the true evaluation of the safety of new "treated" concentrates.

The current status of non-A, non-B viral hepatitis

It is probable that two or more different viruses account for non-A, non-B hepatitis throughout the world, with a third agent causing epidemic hepatitis in India and neighboring countries. NANB virus(es) is the major cause of transfusion-associated hepatitis, and is responsible for roughly 20% of sporadic hepatitis cases. NANB postransfusion hepatitis progresses to chronic hepatitis in half or more of cases. This form of chronic hepatitis, while usually minimally symptomatic, causes progressive liver destruction and eventual cirrhosis in a significant proportion of cases. To date, the NANB virus(es) has not been specifically identified, either serologically or by electron microscopy. When developed, serologic assays will find their most immediate application in the identification of NANB virus carriers among blood donors, thereby being applied to the prevention of post-transfusion hepatitis. No specific therapy is available for NANB virus infection. Gamma globulin is of uncertain prophylactic efficacy.

[Hepatitis non-A, non-B: epidemiologic, clinical, serologic and morphologic aspects]

Hepatitis non-A, non-B (HNANB) is due to one or more transmissible agents, probably viruses. Epidemiologically, HNANB is transmitted predominantly by transfusion of blood or plasma derivatives, and percutaneous inoculation, but a non-percutaneous transmission by the fecal-oral route is also established. However, despite 10 years of intense world-wide research, the transmissible agent, or agents, have not been identified and there are no serological assays for either an antigen or an antibody that can be used to detect this infection. The clinical diagnosis of HNANB remains, therefore, a diagnosis of exclusion mainly of hepatitis A and B, Epstein-Barr virus, cytomegalovirus and drug-induced liver disease. In contrast to hepatitis A and B, the clinical and biochemical course of HNANB tends to be less severe and the proportion of asymptomatic and anicteric cases is higher, but fulminant hepatitis and fatalities also occur. Typically, there is a fluctuating waxing and waning pattern of the serum aminotransferase activities in HNANB. HNANB has a relative high tendency to progress to a chronic stage. The exact frequency of HNANB-induced liver cirrhosis and convincing evidence for an association with hepatocellular carcinoma cannot be assessed, although the persistence of the infectious agent in chronic HNANB and the existence of a chronic asymptomatic carrier state have been proved. By light microscopy there is a broad morphologic spectrum of acute and chronic viral hepatitis, but no single pathognomonic lesion exists that allows a reliable distinction to be made of HNANB from hepatitis A and B. Electron microscopy of liver biopsy specimens of chimpanzees, experimentally infected with HNANB agents, permits the visualisation of cytoplasmic changes, which appear to be specific for infection with HNANB viruses. In human liver biopsy specimens from patients with HNANB, identical ultrastructural cytoplasmic changes could not consistently be demonstrated. In contrast, intranuclear aggregates of spherical and tubular particles measuring 20-29 nm, first described in experimental HNANB in chimpanzees, have been repeatedly demonstrated in acute and chronic HNANB in man. These nuclear particles have been considered as compelling evidence of human HNANB infection. The specificity has been challenged, however, by the demonstration of identical particles in other viral and non-viral hepatopathies and in liver biopsies of healthy volunteers. By immune electron microscopy, a multiplicity of virus-like particles are described in association with HNANB.(ABSTRACT TRUNCATED AT 400 WORDS)

Retrovirus-like particles in hepatocytes of patients with transfusion-acquired non-A, non-B hepatitis

Retrovirus-like particles 60-85 nm in diameter were observed in the cytoplasm of hepatocytes in liver biopsies obtained during the acute and chronic phases of non-A, non-B hepatitis (NANBH) in three patients with transfusion-acquired disease. The particles appeared in dilated endoplasmic reticulum cisternae as well as in enlarged Golgi vesicles. No such particles were seen in hepatocytes in liver biopsies similarly obtained during the acute or chronic phases of NANBH from 11 additional patients with NANBH who did not acquire their disease following blood transfusion. Particle-associated reverse transcriptase activity (peak activity at a density of 1.14 gm/ml) was present in the sera of all three "particle-positive" patients and also in 42% of the "particle-negative" patients. The retrovirus-like particles described here were apparently unrelated to the previously described human T cell lymphocytotropic retroviruses (HTLV), since none of the 14 patients studied had antibodies in their serum directed against antigens of any of the three known HTLVs.

Hepatitis B virus infection in the dental profession

This paper has presented information concerning the degree of hepatitis B infection in the dental profession. This disease is being transmitted from patients to dentists, dental staff members, and their families by both percutaneous and nonpercutaneous routes. Only one of five infections is clinically diagnosed, deeming the patient's medical history an unreliable indication of potential infectivity. All members of the dental profession are at a risk at least three times higher than the general population of contracting this disease and developing the carrier state. Unfortunately, as undiagnosed carriers, members of the dental profession can also unknowingly infect other staff personnel, patients, and family with widespread ramifications.

Unusual viruslike particles in chronic non-A, non-B hepatitis in childhood

Liver biopsies from 16 children with clinical and pathologic evidence of chronic hepatitis have been examined by electron microscopy for cytoplasmic and nuclear changes. Parallel studies by radioimmunoassay on sera from the same patients support the diagnosis of all these cases as non-A, non-B hepatitis (NANB). Ultrathin sections of the liver biopsies demonstrated in one case intranuclear hepatitis B virus-like core particles, 25 nm diameter. In a second biopsy from the same patient, the corelike particles could still be observed. This finding could be used either to support the thesis that a NANB virus is a member of the hepadnavirus group or to reflect the existence of seronegative cases of chronic HBV infection. Furthermore, we have observed in some mononuclear cells from the inflammatory infiltrate of a portal tract, some structures that resemble virus budding. There is a striking similarity between the morphology of these particles (which are enveloped and possess projections) and the ultrastructure of retrovirus.