The effect of metformin treatment during primary influenza infection on heterologous challenge in young and aged mice.

Introduction: Respiratory illnesses like influenza and SARS-CoV-2 disproportionately affect older adults, leading to severe complications and high mortality rates. Age-related immune dysregulation impairs infection responses and hinders recovery. The geroscience hypothesis suggests that targeting biological aging can enhance overall healthspan. Mitochondrial dysfunction and dysregulated nutrient sensing, hallmarks of aging, profoundly affect metabolism and cellular function. Metformin, an FDA-approved diabetes drug, is a candidate anti-aging drug and has been shown to positively impact immune cell function in many contexts. However, the totality of these effects on immune cells remains under investigation. Here, we aim to determine if metformin treatment could improve immune memory responses by utilizing a heterologous flu challenge model. Methods: Young and aged mice were given control or metformin treated chow for 6 weeks prior to being infected with a sublethal dose of H3N2 influenza virus A/HKx31 (X31). Control and treated chow continued until 10 days post infection to examine the effects of metformin on immune memory formation. Mice were then allowed to recover and at 30 days post initial infection and were challenged with a heterologous H1N1 influenza virus A/Puerto Rico/8/34 (PR8). Mice were sacrificed on day 0 (prior to secondary flu challenge), and at 5, 7, 10, and 14 days post-secondary infection to unveil changes in the kinetics of immune responses. Results: Metformin altered only some aspects of immune responses during secondary flu challenge, and more so in young mice compared to aged mice. More specifically, we did not observe improved T cell memory populations in the lungs following primary flu infection in aged metformin treated mice compared to aged control treated mice. Moreover, while aged metformin treated mice had modestly improved weight loss during heterologous challenge, they had transiently increased lung viral load compared to aged control treated mice. Discussion: This suggests that metformin could not overcome the totality of aging to improve T cell memory responses. Thus, while metformin has been shown to have many benefits in a variety of aging conditions, its specific utility in improving age-related declines in immune memory formation during infection is unclear in our studies. More research is necessary to determine how metformin can target aging physiology and T cell function to enhance immune responses, and importantly, understand the limitations of its utility in aging populations.