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

Respiratory syncytial virus tropism for olfactory sensory neurons in mice

The olfactory mucosa, where the first step of odor detection occurs, is a privileged pathway for environmental toxicants and pathogens toward the central nervous system. Indeed, some pathogens can infect olfactory sensory neurons including their axons projecting to the olfactory bulb allowing them to bypass the blood-brain barrier and reach the central nervous system (CNS) through the so-called olfactory pathway. The respiratory syncytial virus (RSV) is a major respiratory tract pathogen but there is growing evidence that RSV may lead to CNS impairments. However, the mechanisms involved in RSV entering into the CNS have been poorly described. In this study, we wanted to explore the capacity of RSV to reach the CNS via the olfactory pathway and to better characterize RSV cellular tropism in the nasal cavity. We first explored the distribution of RSV infectious sites in the nasal cavity by in vivo bioluminescence imaging and a tissue clearing protocol combined with deep-tissue imaging and 3D image analyses. This whole tissue characterization was confirmed with immunohistochemistry and molecular biology approaches. Together, our results provide a novel 3D atlas of mouse nasal cavity anatomy and show that RSV can infect olfactory sensory neurons giving access to the central nervous system by entering the olfactory bulb. Cover Image for this issue: doi: 10.1111/jnc.14765.

Animal Models for Influenza Virus Pathogenesis and Transmission

Influenza virus infection of humans results in a respiratory disease that ranges in severity from sub-clinical infection to primary viral pneumonia that can result in death. The clinical effects of infection vary with the exposure history, age and immune status of the host, and also the virulence of the influenza strain. In humans, the virus is transmitted through either aerosol or contact-based transfer of infectious respiratory secretions. As is evidenced by most zoonotic influenza virus infections, not all strains that can infect humans are able to transmit from person-to-person. Animal models of influenza are essential to research efforts aimed at understanding the viral and host factors that contribute to the disease and transmission outcomes of influenza virus infection in humans. These models furthermore allow the pre-clinical testing of antiviral drugs and vaccines aimed at reducing morbidity and mortality in the population through amelioration of the virulence or transmissibility of influenza viruses. Mice, ferrets, guinea pigs, cotton rats, hamsters and macaques have all been used to study influenza viruses and therapeutics targeting them. Each model presents unique advantages and disadvantages, which will be discussed herein.

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

Because of the marked morphological abnormality of the nasal respiratory epithelium in ferrets recovering from epidemic influenza virus infection, attempts were made to determine whether the anatomical changes were associated with functional changes in the epithelial cells. It was found that on the 7th or 8th day after infection, at which time an immature transitional type of epithelium covers the respiratory area, the cells are resistant not only to reinfection with influenza virus but to a severe physicochemical stimulus supplied by iontophoresis or prolonged irrigation with zinc sulfate. Later, as the ciliated columnar cells return, susceptibility to physicochemical injury returns although resistance to influenza virus persists. The ciliated columnar cells are the ones which are damaged by the physicochemical agent while the deeper cells in the regenerating area remain unaffected. 5 weeks after infection the epithelium is anatomically normal but tissue resistance to zinc sulfate is still present to some degree as evidenced by foci of undamaged cells remaining after ionization. The olfactory epithelium which is undamaged by the PR8 strain of epidemic influenza virus also becomes resistant to ionization after infection. As soon as the respiratory epithelium exhibits any loss of resistance to zinc sulfate the chemical produces complete necrosis of the olfactory area. The refractory state to physicochemical agents exhibited by the regenerating nasal mucosa of the ferret after influenza virus infection is thought to be a non-specific resistant state, significant for a time at least, in the mechanism of immunity to influenza virus.

Eustachian tube histopathology during experimental influenza A virus infection in the chinchilla

The eustachian tubes of 29 influenza A virus-infected chinchillas were examined for histopathologic signs at intervals up to 21 days after inoculation to elucidate the pathologic basis of negative middle ear pressure, which occurs during viral respiratory tract infection in humans. In the animal model, eardrum inflammation and negative middle ear pressure mirrored epithelial damage in the eustachian tube and the accumulation of cellular and mucous debris in the tubal lumen. Epithelial damage was greatest in the proximal two thirds of the tube near the nasopharynx, whereas goblet cell metaplasia and increased secretory activity was greatest in the distal, tympanic one third of the tube. These results provide a morphologic correlate to the development of negative middle ear pressure, and perhaps explain the pathologic basis for purulent otitis media during viral respiratory tract infection.

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.

Immunity to influenza in ferrets. XI. Cross-immunity between A/Hong Kong/68 and A/England/72 viruses: serum antibodies produced by infection or immunization

The degree of immunity due to cross-reactions between antibody to influenza virus A/Hong Kong/1/68 and A/England/42/72 was studied in ferrets. Ferrets were immunized with the viruses by either live infection or by inoculation with inactivated virus vaccines. The vaccines were given with Freund's incomplete adjuvant or were given to ferrets previously infected with influenza virus A/PR/8/34. As a result of these immunizations the animals all produced similar titres of serum HI antibody to the immunizing virus, although the degree of cross-reaction with the other virus strain was variable. After immunization the animals were challenged by infection with an A/Eng/42/72-like virus and their degree of immunity was measured. It was found that the greatest immunity was in ferrets previously infected with the homologous A/Eng/42/72 virus. Animals previously infected with A/HK/68 virus also showed a measurable degree of immunity to A/Eng/42/72 infection, and this was greater than that found in animals given inactivated virus vaccines. The immunity produced by the vaccines was approximately equal, regardless of which vaccine or method of immunization was used. Thus, live infection produced a more effective, broader immunity than did the use of inactivated virus vaccines.

Immunity to influenza in ferrets. V. Immunization with inactivated virus in adjuvant 65

Ferrets infected with influenza virus A2/Hong Kong/3/68 responded with a febrile reaction; the temperature was elevated by 1.0 degrees C. or greater to a level of 40 degrees C. or more. In addition, relatively high titres of virus were recovered from nasal washings taken 3 days after virus infection, serum antibody was produced, increased nasal protein was detected and nasal washings contained both HI and neutralizing antibody. Of four ferrets immunized with 400 CCA units of inactivated influenza virus A2/Aichi/2/68 in saline, only one produced detectable serum HI antibody, and none produced detectable nasal antibody. These ferrets were subsequently found to be susceptible to intranasal infection with influenza virus A2/Hong Kong/3/68. Thus, the temperature response, the titre of virus recovered from nasal washings and the serum HI antibody response found after virus infection was similar to that found after infection of non-immunized ferrets. However, the increase in protein concentration and the titre of HI and neutralizing antibody found in nasal washings after virus infection was detectably less than that found after virus infection of non-immunized ferrets.Four ferrets were immunized with 400 CCA units of inactivated A2/Aichi/2/68 virus in adjuvant 65, and these ferrets produced relatively high titres of serum HI antibody but no detectable nasal antibody. After subsequent virus infection with influenza virus A2/Hong Kong/3/68, these ferrets showed a modified temperature response, reduced titres of virus in nasal washings compared to that found in nasal washings from non-immunized ferrets, no increase in nasal protein and no detectable nasal HI antibody. Thus, immunization with inactivated virus in adjuvant 65 resulted in a significant modification of the response of ferrets to challenge virus; however, the immunity was not complete, and appreciably less than that found after infection with live homologous virus.