Effect of AAV-LAV-BPIFB4 on the clinical frailty index [FI (31 items)] in mice. FI was monitored each month from the inclusion (1st month) up to the 7th month. Injection of AAV-LAV-BPIFB4 (treatment group; solid line) or AAV-GFP (control group; dotted line) was performed at the 3rd and 5th month. (A) FI changes during the study in the whole cohort of mice; (B) FI changes during the study in the cohort of mice subdivided on the basis of the age at inclusion in adult (age range 16-17 months) and old mice (age range 18-23 months). Values of FI are means ± SEM. Statistics to compare FI between treatment and control group was performed using mixed model analysis for longitudinal data (SPSS v. 24.0) including time, treatment, age group and gender as fixed factors and age of mice at the inclusion as covariate; *P<0.05; **P<0.01. BPIFB4 indicates bactericidal/permeability-increasing fold-containing-family-B-member-4; GFP, green fluorescent protein; and LAV, longevity-associated variant.