Robert L. Horswood

Immunoprophylaxis and immunotherapy of respiratory syncytial virus infection in the cotton rat

Human convalescent antiserum to respiratory syncytial virus (RSV) administered intraperitoneally to cotton rats prior to RSV challenge provided near-complete protection from pulmonary infection. Antiserum given subsequent to viral challenge reduced pulmonary viral titers 100-fold or greater within 24 h. Sandoglobulin, a preparation of purified human IgG with high titer of anti-RSV neutralizing activity, produced the same effects as convalescent antiserum. Sandoglobulin was absorbed rapidly and produced a significant therapeutic reduction in virus titer within 3 h. The level of virus reduction in pulmonary and nasal tissues was directly proportional to the neutralizing antibody titer in the cotton rat serum, and was always greater in the lungs than the nose. Animals treated therapeutically with Sandoglobulin had a depressed primary antibody response to infection, but were completely resistant to reinfection with RSV. Histologic examination of pulmonary tissues from Sandoglobulin-treated animals showed no pathologic changes.

DECREASED VIRULENCE AND PROTECTIVE EFFECT OF GENETICALLY STABLE TEMPERATURE‐SENSITIVE MUTANTS OF MYCOPLASMA PNEUMONIAE

Mycoplasrna pneirmoniae is an important cause of pneumonia in children and young adults.1.2 The best estimate of the impact of this organism comes from the surveillance studies of Grayston and Foy in Seattle. They estimated that M. pneirrnoniae caused one pneumonia illness per 1,000 persons per year.2 In addition, several high-risk populations with a considerably higher incidence of disease, such as military recruits and college students, have been identified by us and other investigators.”s Although the organism is sensitive to tetracycline and erythromycin and the severity of disease can be diminished by treatment, these antibiotics do not eradicate M . pneumoniae from the patient’s respiratory tract.G Furthermore, the onset of disease often may be insidious, and considerable debility may occur before the illness is identified correctly. For these reasons it is clear that an effective vaccine is needed. Several inactivated M . pneumoniae vaccines have been developed that were antigenic in Fernald and Clyde, however, have shown that prior infection of the respiratory tract induced significantly more resistance to experimental pulmonary disease in the Syrian hamster than did parenteral inoculation of organisms, although the latter form of immunization stimulated a higher level of serum a n t i b ~ d y . ~ . ~ Hence, it was not surprising that inactivated vaccines that were reasonably effective in stimulating the development of serum metabolisminhibiting antibody in man were found to be less effective in inducing resistance to naturally occurring M . pneumoniae. At most, a 50% protective effect was observed in field trials in which two different vaccines were eval~ated.~,7 In addition, the possibility of sensitization was raised when it was found that vaccinees who failed to develop serum antibody following vaccination experienced more serious disease after experimental challenge than did unvaccinated controls.1° During experimental infection of the hamster with M . pneumoniae, the site of localization of the organism appears to be superficial, involving only the epithelial layer of the respiratory passages.ll Similarly, the organism remains localized to the epithelial surface following infection of organ cultures of human fetal trachea.12 The superficial nature of M . pneumoniae infection in the lower respiratory tract suggests that local immune mechanisms (secretory IgA and/or cell-mediated immunity) in the respiratory tract should be of greater importance than serum antibody in protection from disease.11J2 This inference can also be drawn from the findings of Fernald and Clyde, who observed that resistance to experimental challenge in the hamster was stimulated more effectively by infection of the respiratory tract than by parenteral inoculation of organisms.*~9 In recent studies

INTRAVENOUS IMMUNE SERUM GLOBULIN (IVIG) FOR PROPHYLAXIS AND TREATMENT OF RESPIRATORY SYNCYTIAL VIRUS (RSV)) INFECTIONE IN THE COTTON RAT

Cotton rats receiving IVIG were challenged intranasally 24 hrs later with RSV. These animals and concomitantly infected controls were sacrificed 4 days later. Animals treated with IVIG showed a 100- to 1000-fold reduction in pulmonary virus compared to controls, and a lesser, though highly significant reduction in nasal virus. Cross-homogenization experiments showed the reduced viral titers to be in vivo phenomena, rather than in vitro viral neutralization occurring at the time of tissue homogenization. Several lots of IVIG, all with high neutralizing titers against RSV, were tested in cotton rats, with similar results. Protection was dose-dependent, and was maximal when circulating antibody titers were greater than 1:250. Cotton rats previously infected with RSV were given IVIG to determine if it might alter the course of infection. Animals receiving IVIG as little as 3 hrs prior to sacrifice showed highly significant reductions in pulmonary and nasal viral titers, with the effect greatest in the lungs (geometric mean reduction of over 100-fold). Cross-homogenization experiments showed the reduction to be bona fide in vivo viral neutralization, rather than in vitro artifact. Histologic studies of lungs of IVIG treated animals showed no histopathological abnormalities.

RESPIRATORY SYNCYTIAL VIRUS (RSV) NEUTRALIZING ANTIBODY TITERS HUMAN IMMUNE SERUM GLOBULIN FOR INTRAVENOUS (IVIG) INFUSION

During studies of pharmacokinetics of IVIG in neonates, an infant with a serious lower respiratory tract infection was treated with IVIG and experienced an unexpectedly prompt recovery. Viral cultures of the infants secretions subsequently grew RSV. Analysis of the IVIG lot infused yielded an RSV neutralizing titer of 1:3,276. This prompted a screen of 23 other lots of human IVIG prepared by 3 manufacturers (Cutter Inc., 7 lots; Sandoz Inc., 9 lots; Hyland Laboratories, 7 lots). Eight lots had neutralizing RSV titers exceeding 1:10,240; titers of the others ranged from 1:1,559 to 1:9,344 (ave. 1:4,438). RSV complement fixation (CF) titers generally mirrored the neutralizing titers. However, the RSV CF titers in the 20 lots were generally about 10-fold higher than CF titers to influenza A, B, parainfluenza 1, 2, 3 and adenoviruses. Reinfection of humans by RSV is common. Coupled with the annual RSV outbreaks in most communities, recurrent reexposure probably accounts for the high titers in the IVIG lots. The data suggested that IVIG may be suitable for prophylaxis or treatment of RSV infections. Utilization of high titered RSV-IVIG preparations for passive immunotherapy of RSV disease is currently being examined in cotton rat and owl monkey models.