STUDIES ON THE NASAL HISTOLOGY OF EPIDEMIC INFLUENZA VIRUS INFECTION IN THE FERRET : II. THE RESISTANCE OF REGENERATING RESPIRATORY EPITHELIUM TO REINFECTION AND TO PHYSICOCHEMICAL INJURY

Animal models for influenza virus pathogenesis, transmission, and immunology

In humans, infection with an influenza A or B virus manifests typically as an acute and self-limited upper respiratory tract illness characterized by fever, cough, sore throat, and malaise. However, influenza can present along a broad spectrum of disease, ranging from sub-clinical or even asymptomatic infection to a severe primary viral pneumonia requiring advanced medical supportive care. Disease severity depends upon the virulence of the influenza virus strain and the immune competence and previous influenza exposures of the patient. Animal models are used in influenza research not only to elucidate the viral and host factors that affect influenza disease outcomes in and spread among susceptible hosts, but also to evaluate interventions designed to prevent or reduce influenza morbidity and mortality in man. This review will focus on the three animal models currently used most frequently in influenza virus research - mice, ferrets, and guinea pigs - and discuss the advantages and disadvantages of each.

IMMUNITY TO YELLOW FEVER ENCEPHALITIS OF MONKEYS AND MICE IMMUNIZED BY NEURAL AND EXTRANEURAL ROUTES

Monkeys and mice surviving cerebral infection with yellow fever virus of relatively avirulent strains have been found to resist maximal intracerebral doses of yellow fever virus of a highly neurotropic strain. Such animals, however, do not resist more than very small doses of intracerebrally inoculated virus of Eastern equine encephalomyelitis. Animals immunized by extraneural routes, on the other hand, are not uniformly resistant to neural infection with neurotropic yellow fever virus. Monkeys which have undergone systemic infection with virus of the avirulent 17D strain or of several jungle strains resist only small intracerebral doses of neurotropic virus; while mice, even when possessed of very high serum-antibody levels as the result of intraperitoneal hyperimmunization, manifest only an irregular resistance to intracerebral challenge inocula. The difference in the resistance of neurally and extraneurally immunized animals is not related to similar differences in the levels of protective antibody in the sera. Indeed, the average of the serum-antibody titers of the hyperimmune mice is several times that of the intracerebral immunes. A possibly significant relation does exist, however, between the resistance of mice to neural infection and the content of protective antibody in the brain. The protective activity of suspensions of brains from mice surviving cerebral infection was found to be several times that of brain suspensions from the hyperimmunized animals. It is concluded that the superior resistance to neural infection of animals whose immunity results from a previous non-fatal infection of the nervous system is effected by a specific local mechanism which is based at least in part upon an increased concentration of antibody in the cerebral tissue.

STUDIES ON THE NASAL HISTOLOGY OF EPIDEMIC INFLUENZA VIRUS INFECTION IN THE FERRET : III. HISTOLOGICAL AND SEROLOGICAL OBSERVATIONS ON FERRETS RECEIVING REPEATED INOCULATIONS OF EPIDEMIC INFUENZA VIRUS

A study of the respiratory mucous membrane was made in the turbinates of ferrets which had received repeated inoculations of influenza virus. There was no evidence that persistent immunity is related to the presence of a structural modification of the respiratory epithelium. In fact, the respiratory epithelium in fully immune animals differs histologically only in minor respects from that of the normal, untreated ferret. On the other hand, a functional difference exists between the normal and the previously infected animals as evidenced by a marked acceleration of the repair process in the latter. Serological studies at the time of reinfection, 4 months or more after the previous attack, indicate that a relation exists between the height of antibody titer and resistance. The degree of immunity is probably a product of serological immunity and the rate of tissue repair. The implications of these studies to the problem of influenza in man are discussed.

Towards a quantitative understanding of the within-host dynamics of influenza A infections

Although the influenza A virus has been extensively studied, a quantitative understanding of the infection dynamics is still lacking. To make progress in this direction, we designed several mathematical models and compared them with data from influenza A infections of mice. We find that the immune response (IR) plays an important part in the infection dynamics. Both an innate and an adaptive IR are required to provide adequate explanation of the data. In contrast, regrowth of epithelial cells did not seem to be an important mechanism on the time scale of the infection. We also find that different model variants for both innate and adaptive responses fit the data well, indicating the need for additional data to allow further model discrimination.

Effect of specific humoral immunity and some non-specific factors on resistance of volunteers to respiratory coronavirus infection

Thirty-three volunteers were inoculated intranasally with coronavirus 229 E, and their responses monitored by antibody rises, symptomatology and virus excretion. These were related to their pre-trial immune status as indicated by concentrations of specific antibodies and non-specific proteins in serum and nasal washings. Both circulating and local specific antibodies were associated with protection from infection and disease, but only specific IgA antibodies of either type appeared to shorten the period of virus shedding. Although total secretory IgA was significantly associated only with reduction of symptoms, total protein in nasal washings appeared to protect against infection also, indicating that other locally produced proteins, not identified, may be associated with resistance. Two of the many factors which may affect the concentration of circulating and local protective proteins and thus influence the outcome of virus inoculation, namely, sex of the volunteer and the interval since the previous cold, were examined. Male volunteers or volunteers who had had evidence of a recent respiratory infection were less likely to be infected, but if they were infected, they had lower clinical scores and stopped shedding virus earlier than the rest. These groups possessed higher concentrations of specific antibodies and non-specific proteins in their pre-challenge sera and/or nasal washings. The significance of these findings is discussed.

The search for the ideal influenza vaccine

The history of the development of influenza virus vaccine is traced from its origin with experimental studies of influenza virus in ferrets and mice and the first trials in man. Knowledge of the basis of immunity to the viruses in experimental animals and in man has grown steadily over the years and has been essential to successful immunization. Virus variation affecting the surface antigens of the virus is seen as the principal obstacle to the application of vaccines in man. So significant are the changes occurring during antigenic drift that former concepts of a polyvalent vaccine cannot provide a solution of the problem of the composition of vaccines. Disrupted virus vaccines appear to provide the answer to the prevention of vaccine reactions.

Olfactory and vomeronasal deafferentation of male hamsters: histological and behavioral analyses

Deafferentation of the vomeronasal system by cutting the vomeronasal nerves severely impaired mating behavior in 44% of male hamsters over a 1--2 month period of postoperative testing, but the remaining males mated normally after the surgery. Damage to the main olfactory bulbs, concomitant to vomeronasal nerve cuts, did not account for this behavioral difference. Subsequent deafferentation of olfactory system by intranasal infusion of zinc sulfate solution (5 g ZnSO4--7H2O in 95 ml 0.5% NaCl) had no effect on intromission or ejaculation latencies of sham vomeronasal cut males but eliminated mating behavior 2 days after treatment in males with bilateral vomeronasal nerve cuts. Some of these males recovered the behavior in 1--3 weeks of post zinc sulfate testing. Histological analyses of the olfactory mucosa in 7 males on day 2 after zinc sulfate showed that 89--97% of the mucosa had been destroyed in 6 out of 7 of the males and 78% in the seventh. We conclude that destruction of the vomeronasal system irreparably reduces arousal necessary for mating in some hamsters but in other males sufficient arousal for this behavior to occur is mediated through the olfactory system, presumably in conjunction with other sensory inputs. Subsequent removal of the olfactory input in these animals eliminates the behavior.

Light and electron microscopic study of the degeneration and early regeneration of olfactory epithelium in the mouse

Degeneration and early regeneration of olfactory epithelium from two strains of mice was studied at the light and electron microscopic levels from 12 hours to 3 days following nasal irrigation with 1% aqueous solution of zinc sulfate (ZnSO4) (a compound known to selectively damage olfactory epithelium). Distinct patterns of degeneration and stages of regeneration were evident following treatment. During the first 24 hours after treatment three progressive manifestations of the degenerative process were seen: (1) a relatively mild condition which was characterized by surface irregularities produced by cell protrusions, highly vacuolated cytoplasm, presence of large lysosome-like bodies and prominent intercellular spaces, (2) a more severe condition in which large areas of the epithelium were detached from the basement membrane cellular debris was present in the nasal chamber, and (3) a condition of total or near-total denudation of the epithelium of olfactory mucosa. The basal lamina was continuous and intact in most regions and the integrity of the subadjacent connective tissue was mostly well-preserved. Nerve bundles of the fila olfactoria were noted in varying degrees of degeneration during the course of the experiment. The most advanced neural degeneration was seen 24 to 72 hours after treatment. Onset of regeneration was suggested by the appearance of a simple squamous layer of cells above the basement membrane 48 to 72 hours after treatment. In addition to the simple epithelium a stratified epithelium consisting of two to four cell layers was also observed at this time. Glandular cells, containing secretory granules identical to those in Bowman's glandular cells, were noted in an apparent process of migration from the lamina propria into the the stratified epithelial layer. The last mentioned observation supports the proposition that new supportive epithelial cells originate from cells of Bowman's gland.

Ultrastructural study of mouse olfactory epithelium following destruction by ZnSO4 and its subsequent regeneration

Regeneration of olfactory epithelium was studied at the ultrastructural level in two different strains of mice (C57B1/6J and SWR/J) following destructive changes induced by nasal irrigation with 1% aqueous solution of zinc sulfate (ZnSO4). Three epithelial conditions were seen during the first four to 14 days following treatment: a "surface alteration" state characterized by changes at the apical ends of olfactory and columnar supporting cells, a "squamous-metaplastic" state, and a simple-squamous" state. In the latter two conditions, thought to represent regenerative states, cells contained inclusions similar to secretion granules in Bowman's glands. This observation supports earlier suggestions that new epithelial cells originate from these glands. Later regenerative changes consist of differentiation of new sensory cells and columnar supporting cells in an initially disorganized epithelium. This epithelium then changes to a normal, pseudostratified organization. Regeneration of the epithelium begins earlier and proceeds more rapidly in the SWR/J strain than C57B1/6J mice. Possibly, a genetic difference exists between strains. After 72 days the epithelium in both strains had regenerated.