The major antigenic properties of rotaviruses—group, subgroup, and serotype—are determined by the viral capsid proteins. Rotavirus has seven major groups (A-G); most human strains belong to group A, although groups B and C have occasionally been associated with human illness. The product of the 6th gene of group A rotaviruses encodes VP6, the most abundant viral protein, which is the major determinant of group reactivity, the target of common diagnostic assays, and contains the antigen used to further classify rotaviruses into subgroups I and II. The outer capsid proteins, VP7, the glycoprotein or G-protein (encoded by gene 7, 8, or 9, depending on the strain), and VP4, the protease-cleaved or P-protein (encoded by gene segment 4), determine the serotype specificity and form the basis of the binary classification (G and P type) of rotaviruses. Both G and P proteins induce neutralizing antibodies and may be involved in protective immunity.
Fourteen G serotypes of rotavirus, 10 of which occur in humans, have been defined by cross-neutralization studies with polyclonal animal serum samples; these serotypes correlate with antigenic specificities of the VP7 glycoprotein. The characterization of P serotypes has been difficult because adequate reagents are not available. Eight P serotypes of human rotaviruses have been characterized. Additional VP4 gene variants have been identified, so ultimately the number of P serotypes may exceed 20. Theoretically, 80 different strains of rotavirus could result from various combinations of the known 10 G and 8 P serotypes of human rotaviruses. For vaccine development purposes, it is fortunate that only four common strains (P[8]G1; P[8]G3; P[8]G4; and P[4]G2) of rotavirus predominate globally (Figure 4) (11). However, the prevalence of rotavirus strains varies considerably from one geographic area to another, and unusual strains are common in several developing countries (e.g., unusual P[6] strains, including those with serotype G9 specificity, accounted for 9.5% of all rotaviruses from a multicenter collection in India)
In infants and young children, neutralizing antibodies directed primarily against the G serotype of the infecting strain (homotypic response) develop after primary infection with rotavirus (18). Repeat rotavirus infections elicit both a homotypic and heterotypic (against strains with different G serotypes) antibody response. Protection against rotavirus diarrhea correlates with serum antibody titers following natural infection of young children, and infected children are more protected against reinfection with similar rather than different G serotypes. A protective role of placentally transferred maternal antibody among infants < 3 months of age has also been speculated since rotavirus disease is uncommon in this age group. However, serum neutralizing antibody responses among vaccine recipients have sometimes correlated poorly with protection from disease; therefore, the exact role of serum antibody in protection against disease remains unclear.
Genome Variability
Rotavirus is a highly variable virus even within the subset of those that are infective to humans. [4] Rotaviruses are usually categorized into seven groups A-G, with subgroups I and II based on the VP6 protein. [8] Within these groups, A, B, and C are infective to Humans. [19]
Rotavirus is further categorized into G and P serotypes. The G serotype is specified by the glycoprotein VP7 of the outer capsid, which is coded by viral genes 7, 8, and 9. [8] The P serotype is specified by protein VP4, also on the outer capsid. It is a protease cleaved protein coded by gene 4 of the virus genome. [8] The most common G serotypes currently are G1, G2, G3, G4, and G9, with G1 being most prevalent and G9 the fastest emerging worldwide. [7] [8] [10] [12] Common P serotypes are P1a, P1b, and P8. [13] [6] [4]
Monovalent “Jennerian” Vaccines
Initial development of rotavirus vaccines was based on the Jennerian approach, which involved the use of a live, attenuated, antigenically related virus derived from a nonhuman host (21). This approach was prompted by studies indicating that animal and human rotaviruses shared a common group antigen and that experimental animals immunized with animal strains of rotavirus had a significantly lower risk for illness and viral shedding when subsequently challenged with human rotaviruses. Furthermore, neutralizing antibodies to human rotavirus serotypes in the animal models indicated the potential for cross-protection.
Bovine Vaccines
The first two Jennerian vaccines were developed with bovine rotavirus strains RIT4237 and WC3. The WC3 strain was passaged in cell culture less than RIT4237 and was developed because of concern that excessive passaging of the RIT vaccine might cause overattenuation and diminished efficacy. RIT4237 and WC3 were nonreactogenic and immunogenic when administered to infants 2 to 18 months of age. However, the protection conferred by both vaccines varied greatly in efficacy studies, 0% to 76% against any rotavirus diarrhea and 0% to 100% against severe disease (22–32). A well-defined correlate of protection was not identified, and reasons for the variable efficacy were unknown, although late age at vaccination, timing of vaccination with respect to the onset of the rotavirus season, and variations in the strength and number of doses of the vaccine were proposed as contributing factors. Both vaccines performed less well in developing than in industrialized countries, possibly because of interference by other enteropathogens or inadequate surveillance during follow-up.
Rhesus Vaccine
The third Jennerian vaccine was developed with rhesus rotavirus strain MMU18006, which shares neutralization specificity with human rotavirus G3 strains. Besides sharing antigenic specificity with an epidemiologically important human rotavirus serotype, MMU18006 was suitable for vaccine development because it grew efficiently in cell culture. As in the bovine rotavirus-based vaccines, MMU18006 was safe and immunogenic, although in some trials, one third of infants became febrile 3 to 4 days after vaccination. The reactogenicity of MMU18006 was particularly high in two studies in Finland and Sweden in which 64% and 79% of infants, respectively, became febrile. Most children with febrile responses were >5 months of age; lack of passively transferred maternal antibody might have contributed to the high reactogenicity of the vaccine. As in the RIT4237 and WC3 vaccines, the protective efficacy of MMU18006 in field trials was quite variable, 0% to 60% against any rotavirus diarrhea and 0% to 85% against severe rotavirus diarrhea (33–39).
Reassortant “Modified Jennerian” Vaccines
The greatest efficacy of MMU18006 was observed in a Venezuelan trial in which the rotavirus strain circulating in the community (G3) was the same serotype as the vaccine strain, which suggested that serotype-specific immunity against each of the epidemiologically important strains of human rotaviruses may be required for maximum protection. Similar observations in vaccine challenge cross-protection studies in animals initiated the development of vaccines that used a modified Jennerian approach in which animal-human reassortants expressing VP7 proteins of serotypes 1 through 4 were used as the immunogens.
Rhesus-Human Reassortant Vaccines
Rhesus-human reassortants were generated by coinfecting cell cultures with rhesus rotavirus (RRV) strain MMU18006 (G serotype 3) and human rotavirus strains D (G serotype 1), DS-1 (G serotype 2), and ST3 (G serotype 4). Selection pressure (induced by the addition of neutralizing antibody to VP7 of RRV) produced reassortant strains D x RRV, DS-1 x RRV, and ST3 x RRV, each of which possessed the VP7 gene from HRV serotype 1, 2, or 4 and the other 10 genes from RRV (Figure 6) (40). Because vaccines made from the individual reassortants were safe and immunogenic, RRV-TV was developed incorporating each of the three reassortants and MMU18006 to provide coverage against the four common VP7 serotypes of rotavirus.
RRV-TV testing was initiated at an inoculum of 104 PFU of each of the four viruses (i.e., at 4 x 104 PFU) and completed at 105 PFU, the dose submitted for licensure (i.e., at 4 x 105 PFU). In most trials, vaccine was administered orally in three doses separated by at least 3 weeks to optimize the immune response to the component antigens; immunization was completed by the age of 6 to 7 months. Because the vaccine virus strains are acid labile, they are administered with 2.5 ml of citrate-bicarbonate buffer.
Bovine-Human Reassortant Vaccines
Bovine-human reassortant rotavirus vaccines include a tetravalent WC3 rotavirus reassortant vaccine with genes coding for the VP7 of three major serotypes of rotavirus (G1, G2, and G3) and W179-4, a human VP4 reassortant with P[8] specificity. Theoretically, this vaccine should induce antibodies broadly reactive to the three common serotypes of rotavirus sharing P [8] specificity, thereby increasing the protective efficacy of this vaccine. In an efficacy trial of a three-dose regimen of the WC3 reassortant vaccine, protection was 67% against all rotavirus diarrhea and 69% against severe rotavirus diarrhea (48).
Other Candidate Vaccines
In clinical trials, no Jennerian vaccine has provided complete protection against rotavirus diarrhea; as a result, several non-Jennerian candidate vaccines are being developed. Vaccines based on neonatal, cold-adapted, and attenuated human strains of rotavirus are under evaluation (49). Other approaches, such as the use of baculovirus-expressed viruslike particles or naked DNA vaccines, are also being used to develop candidate rotavirus vaccines (50,51).
The Ever-Changing Landscape of Rotavirus Serotypes
(The Pediatric Infectious Disease Journal:Volume 28(3) Supplement March 2009pp S60-S62)
Abstract:
Rotavirus is a double-stranded RNA virus that is characterized by substantial genetic diversity. The various serotypes of rotavirus have been determined by the presence of neutralizing epitopes on the outer capsid of the protein shell. At present, 5 rotavirus serotypes (G1, G2, G3, G4, G9) are the predominant circulating strains, accounting for approximately 95% of strains worldwide, although there is considerable geographic variability. Incidence rates for various serotypes also vary temporally with seasonal and year-to-year fluctuations. Unusual serotypes are generally uncommon, but new serotypes can emerge. In particular, G9[P8], a reassortment virus, was first identified in 1983 and in the last 10 to 15 years has become widely distributed worldwide. Indeed, G9[P8] has become highly prevalent in many countries in Europe and Australia, with somewhat lower incidence rates in South America, Africa, and Asia. The heterogeneity and ever-changing epidemiology of rotavirus underscores the need for continued surveillance to ensure that vaccination programs provide optimal protection.
Three oral RV vaccines are currently licensed, a human monovalent live attenuated RV strain, RotarixTM, a pentavalent live bovine-human reassortant vaccine, RotaTeqTM, and a lamb-derived monovalent live attenuated strain, LLR, which is only being used in China.
ROTARIX® (Rotavirus Vaccine, Live, Oral)
ROTARIX is a vaccine indicated for the prevention of rotavirus gastroenteritis caused by G1 and non-G1 types (G3, G4, and G9) in infants and children.
ROTARIX (Rotavirus Vaccine, Live, Oral), for oral administration, is a live, attenuated rotavirus vaccine derived from the human 89-12 strain which belongs to G1P [8] type. The rotavirus strain is propagated on Vero cells. After reconstitution, the final formulation (1 mL) contains at least 106.0 median Cell Culture Infective Dose (CCID50) of live, attenuated rotavirus.
RotaTeq (Rotavirus Vaccine, Live, Oral, Pentavalent) Oral Solution
RotaTeq is indicated for the prevention of rotavirus gastroenteritis in infants and children caused by the G1, G2, G3 and G4 serotypes contained in the vaccine.
RotaTeq, 2 mL for oral use, is a ready-to-use solution of live reassortant rotaviruses, containing G1,G2, G3, G4 and P1A[8] which contains a minimum of 2.0 – 2.8 x 106 infectious units (IU) per individual reassortant dose, depending on the serotype, and not greater than 116 x 106 IU per aggregate dose.
RotaTeq is a live, oral pentavalent vaccine that contains 5 live reassortant rotaviruses. The rotavirus parent strains of the reassortants were isolated from human and bovine hosts. Four reassortant rotaviruses express one of the outer capsid proteins (G1, G2, G3, or G4) from the human rotavirus parent strain and the attachment protein (serotype P7) from the bovine rotavirus parent strain. The fifth reassortant virus expresses the attachment protein, P1A (genotype P[8]), herein referred to as serotype P1A[8], from the human rotavirus parent strain and the outer capsid protein of serotype G6 from the bovine rotavirus parent strain…
Monovalent Lamb Vaccine
The only vaccine that is currently licensed is in use in China. Developed by Zhi-Sheng Bai at the Lanzhou Institute, the attenuated monovalent vaccine is based on a strain of lamb rotavirus. Similar to Rotarix®, the lamb rotavirus strain was attenuated by passage in cell culture.
The lamb vaccine is classified as serotype P[10], G12. It is delivered as a single dose, between the ages of 2-24 months.
The vaccine has been proven to be safe and immunogenic (with 61% of vaccinees developing neutralizing antibody responses), but efficacy results of the Phase II trails have yet to be published.[6]
Additional Information about the vaccines:
The first RV vaccines to be tested in humans were the live bovine strains RIT4237 (P[1]-G6) and WC3 (P[5]-G6), and the live simian strain RRV (P[3]-G3), which are attenuated for humans and could be administered by the oral route. The three strains induced neutralizing antibodies in a majority of infants but showed inconstant capacity to protect against RV disease.
In China, a lamb-derived monovalent (P[12]-G10) live-attenuated, 3-dose oral vaccine, was developed by the Lanzhou Institute of Biomedical Products and is used in the private sector. The vaccine is reported to induce neutralizing antibody responses in 60% of vaccinees but its efficacy is not precisely known since it was not tested against placebo in a controlled Phase III trial [135] .
A human P[8]-G1 RV strain, RIX 4414, which was isolated from the stools of a sick 15-month old boy in the USA, was attenuated by multiple passages in cell culture, plaque-purified and passaged again in Vero cells. The strain was developed as a 2-dose monovalent oral vaccine by AVANT Immunotherapeutics then licensed to GlaxoSmithKline Biologicals. The vaccine (RotarixTM) showed 70%-85% protective efficacy against severe disease, including that due non-G1 serotypes [149, 150]. It now has been tested in more than 60 countries in Latin America, Africa, Asia and Europe. A large, multicentered safety trial on 63 225 infants between 6 and 14 weeks of age in Latin America and Finland confirmed the initial safety data and indicated no increased attributable risk of intussusceptions (IS) in the high-risk period up to 30 days post any dose [151]. The vaccine was first licensed in 2004 in Mexico and the Dominican Republic and has now been licensed in many countries worldwide. It also has been prequalified by WHO for procurement by UNICEF and the UN Vaccine Fund. Additional Phase IIb and III trials are in progress in South Africa, Malawi and Bangladesh to determine if the vaccine will work well in children from poor settings in developing countries, if it can be administered with the oral polio vaccine without interference, and whether it can safely be administered to HIV positive infants. Final results are due in 2009.
Another human RV strain, RV3 (P2[6]-G3), isolated from newborns at the Royal Hospital in Melbourne, Australia [152] is also developed as a candidate live oral vaccine. A small Phase II study with three doses of 105 pfu of the vaccine indicated relatively low immunogenicity in infants as measured by serum IgA levels [153] . However, the vaccine recipients who developed an immune response were protected against clinical disease in the following year. Strategies to increase the potency of the vaccine are under study with a vaccine producer in Indonesia.
6.3.2 Live reassortant RV strains
Efforts also were made to develop human-animal reassortant RV strains containing the VP7 or VP4 RNA segment from a human RV strain to provide the required antigenicity and the other 10 RNA segments from a simian or a bovine strain to provide the attenuated phenotype [154, 155].
A tetravalent rhesus-human reassortant RV vaccine, RRV-TV, was initially developed at the NIH, Bethesda, using the simian RRV strain (G3) mixed with three human-simian reassortant strains of G types 1, 2, and 4, respectively. The vaccine (RotaShieldTM, Wyeth-Lederle Vaccines, USA) was shown to provide 48–68% protection against any RV disease and 64–91% protection against severe disease [156] . It was introduced in August 1998 on the market in the USA and administered in a three dose schedule to over 600 000 infants within the following year, until an unexpected adverse event, intussusception (IS), was found to occur in a significant number of cases within two weeks after administration of the first two doses of vaccine, leading to its eventual withdrawal [157] . The risk of IS, initially targeted at 1 in 2500 children immunized, has now been reassessed as 1 in 10 000. Its occurrence led to a very thorough safety assessment of the following generation of live oral RV vaccines (viz RotarixTM and RotaTeqTM), with sample sizes in excess of 60 000 subjects. The original RotashieldTM vaccine has now been licensed to a biotech company, BIOVIRx Inc., USA.
A pentavalent human-bovine reassortant vaccine, RotaTeqTM, was prepared by Merck Research Co., Pennsylvania, by reassortment between the naturally attenuated bovine RV strain WC3 and five different human RV strains of serotypes G1, G2, G3, G4 and P[8], respectively. The live-attenuated, 3-dose oral vaccine, was tested in a large safety and efficacy trial in Finland and the USA on more than 70 000 children who were carefully monitored for 2 weeks after each immunization for risks of IS. The vaccine was found to be totally safe and to elicit 74% protection against any G1-G4 RV gastroenteritis through the first RV season after vaccination [158]. Vaccination reduced doctor visits for RV diarrhoea by 86% and hospitalizations and emergency department visits by 94.5%. The vaccine was shown not to interfere with the immunogenicity of a combined Hib, DTP, HepB, conjugated pneumococcal and inactivated polio vaccine, nor with concomitant administration of the oral polio vaccine [159]. RotaTeqTM was licensed in February 2006 in the USA and subsequently in many countries worldwide. It officially was recommended for the routine immunization of children in the USA after active surveillance showed only three cases of IS among more than 100 000 vaccinated infants. It also has been included into national vaccination programs in several countries. A Phase III trial is ongoing in African countries (Mali, Ghana, and Kenya). Results are expected end of 2009.
An alternative multivalent bovine-human reassortant oral vaccine was developed by the National Institute of Allergy and Infectious Diseases (NIAID, NIH, Bethesda), based on the attenuated bovine strain UK reassorted with the five most common RV serotypes in humans, G1-G4, G8 and G9 [156] . Phase II data showed a good immune response and no adverse interference with concomitantly administered childhood vaccines. A non-exclusive license for production of the vaccine has been granted to vaccine producers in Brazil, China and India.
Finally, a naturally occurring human–bovine, neonate-derived RV strain, 116E (P[10]-G9), which was isolated from a nosocomial outbreak of asymptomatic infection in New-Delhi, is under development by Bharat Biotech Ltd in India [148, 160]. A similar strain, I321 (P[11]-G10), was found not to be immunogenic [161] .
6.3.3 Other RV vaccine approaches
New RV vaccine approaches include an inactivated virus vaccine [162] , DNA vaccines [163] , a VP6 subunit vaccine [164, 165] and virus-like particles (VLPs) expressed in a baculovirus system [166-168]. Depending on the number of viral proteins expressed, the complexity of the VLPs can vary from mono-layered (VP2-VLPs) to double-layered (VP2/6 VLPs) or triple- layered VLPs (VP2/6/7/4 VLPs).
Filed under: Rotavirus | Tagged: Rotavirus | Comments Off on Rotavirus Strains
Exploring Vaccines 