Biochemical and epidemiological characterization of serogroup C rotavirus

Abstract

Rotaviruses are classified into seven serogroups (A-G) on the basis of antigenic and genetic properties. Group A rotavirus is the most common cause of diarrhea in infants and young children in both developing and developed countries. Group C rotaviruses were first detected in pigs but have subsequently been found sporadically in children and adults with diarrheal illness in many parts of the world. The difficulties to grow group C rotavirus has hampered the biochemical characterization and to date the proteins of group C rotavirus have only been partly identified and characterized. Furthermore, the incidence and clinical significance of group C rotavirus in gastrointestinal illness remains unclear.

The polypeptide synthesis and morphogenesis of group C rotavirus (strain AmC-1/Cowden) adapted to growth in a continous swine testicular cell line was examined. SDS-PAGE of infected 35S methionine labeled cell lysates revealed nine viral polypeptides and six of the polypeptides were subsequently demonstrated to be structural components of the virus particle. Two glycoproteins were described - a homologue to the vp7 outer capsid protein of group A rotavirus and a nonstructural glycoprotein, most likely a homologue to the enterotoxin nsp4 of group A rotavirus. In addition a structural protein equivalent to the group A outer capsid protein vp4 was described for the first time. This protein was recognized by a monoclonal antibody displaying both neutralizing and hemagglutination inhibition capacity, which indicated the discovered protein to function as an attachment protein. Morphogenesis studies revealed a process, as described for group A rotavirus, with formation of viroplasms involved in the assembly of single capsid particles, which subsequently budded into the endoplasmic reticulum and a final maturation into double capsid particles. We demonstrated that vp6 alone has the capacity to organize into viroplasmic structures, while additional viral proteins were required for the formation of single capsid particles, as demonstrated by coexpression of the major core protein vp2 and the inner capsid protein vp6 followed by the assembly of virus-like particles.

Since group C rotaviruses have not previously been described in Sweden, we conducted a seroepidemiological study to establish the prevalence of human group C rotavirus infections. Among 160 sera randomly collected from eight different age groups, an average of 38 % contained antibodies to group C rotavirus, with the highest seroprevalence (45%) among individuals aged 11-30 years. The protein specific antibody responses were directed against the major structural proteins vp2, vp4, vp6 and vp7 and the non-structural protein nsp2. We took advantage of a one-year prospective study to investigate the role of group C rotavirus infections in acute diarrhea in Swedish adults. Rotaviruses constituted 3 % of the identified infections and in 35 % of these cases was group C rotavirus demonstrated. The outer capsid protein vp7 of the identified isolates demonstrated a very high sequence homology (99. 1 - 100 % identity at the amino acid level). Clinical symptoms including diarrhea, vomiting, fever and abdominal pain were generally milder than for group A rotavirus infections.