Lytic phage cocktail elicits route-dependent humoral immunity in rats, strongest via IP administration.

Antimicrobial resistance (AMR), particularly from multidrug-resistant Klebsiella pneumoniae (K. pneumoniae), poses a significant global health threat, driving the search for alternatives to conventional antibiotics. Phage therapy, utilizing bacteriophages to selectively target and lyse bacteria, offers a promising approach. However, the host's immune response to repeated phage exposure, and how different administration routes influence these responses, remains a critical knowledge gap. Understanding these interactions is essential for optimizing phage therapy protocols and ensuring long-term efficacy.

Researchers repeatedly administered a cocktail of four lytic phages targeting multidrug-resistant Klebsiella pneumoniae to healthy rats. The phage cocktail was delivered via four parenteral routes: intraperitoneal (IP), intramuscular (IM), intravenous (IV), and subcutaneous (SC). Humoral immune responses were quantified by measuring anti-phage immunoglobulins (IgM, IgG, IgA) and neutralising antibodies. Systemic inflammatory responses were assessed by quantifying key cytokines, including TNF-α, IL-1β, IL-6, and IL-10, across all treatment groups.

Repeated phage administration elicited distinct, route-dependent humoral immune responses. Neutralising antibody levels peaked by day 14 in the IP, IM, and IV groups, reflecting early IgM responses. A second, later peak in antibody levels was observed after the seventh dose, administered 37 days into the study.

The IP route consistently triggered the strongest and most sustained antibody response, particularly for IgG levels, indicating a robust and prolonged immune activation. Conversely, responses following SC administration were notably weaker and delayed compared to other routes. IgA levels remained low across all administration routes. Cytokine analysis revealed only mild, transient fluctuations in TNF-α, IL-1β, IL-6, and IL-10, with no evidence of sustained systemic inflammatory activation, suggesting good tolerability.