Cross-reactive neutralizing antibodies directed against pandemic H1N1 2009 virus are protective in a highly sensitive DBA/2 mouse influenza model

Abstract

Our ability to rapidly respond to an emerging influenza pandemic is hampered somewhat by the lack of a susceptible small-animal model. To develop a more sensitive model, we pathotyped 18 low-pathogenic non-mouse-adapted influenza A viruses of human and avian origin in DBA/2 and C57BL/6 mice. The majority of the isolates (13/18) induced severe morbidity and mortality in DBA/2 mice upon intranasal challenge with 1 million infectious doses. Also, at a 100-fold-lower dose, more than 50% of the viruses induced severe weight loss, and mice succumbed to the infection. In contrast, only two virus strains were pathogenic for C57BL/6 mice upon high-dose inoculation. Therefore, DBA/2 mice are a suitable model to validate influenza A virus vaccines and antiviral therapies without the need for extensive viral adaptation. Correspondingly, we used the DBA/2 model to assess the level of protection afforded by preexisting pandemic H1N1 2009 virus (H1N1pdm) cross-reactive human antibodies detected by a hemagglutination inhibition assay. Passive transfer of these antibodies prior to infection protected mice from H1N1pdm-induced pathogenicity, demonstrating the effectiveness of these cross-reactive neutralizing antibodies in vivo.

FIG. 1.

Virus titer in lungs of C57BL/6 (•) and DBA/2 (▿) mice at 2 and 7 days postinoculation with 10⁴ EID₅₀ of A/shorebird/Delaware/22/2006 (H7N3), A/blue winged-teal/Alberta/271/2007 (H10N5), and A/California/4/2009 (H1N1pdm). *, P < 0.01. Infectious titer values on the y axis represent EID₅₀/ml for H7N3 and H10N5 viruses and TCID₅₀/ml for H1N1pdm.

FIG. 2.

Human cross-reactive 2009 pandemic H1N1-neutralizing antibodies are functional in vivo. Human serum pools with (H1 pos) or without (H1 neg) a detectable cross-reactive 2009 pandemic H1N1 (A/California/4/2009)-neutralizing antibody titer were injected intraperitoneally 24 h prior to intranasal lethal challenge. Control mice were injected with PBS or convalescent-phase serum obtained from 2009 pandemic H1N1 virus-infected ferrets. Survival was monitored for 21 days, and weight loss was monitored for 16 days. Data shown in panel A are the cumulative results of two experiments, and those in panel B represent the average weight loss of one indicative experiment. (A) ***, P < 0.001 compared to results for other groups; **, P < 0.01 compared to results for PBS control group and P < 0.05 compared to results for H1-negative group; *, P < 0.01 compared to results for PBS control group. (B) *, P < 0.01 compared to results for other groups..