Outsourced Immunity: How Eco-Evolutionary Strategies May Dictate Divergent Pro- and Anti-Inflammatory Bacteriophage Profiles

Traditionally perceived in classical microbiology mainly as lytic agents biologically inert toward the eukaryotic host, bacteriophages are now increasingly recognized for their multifaceted interactions with the mammalian immune system. Here, we outline a conceptual framework, termed “outsourced immunity”, suggesting that phage-induced immunomodulation may act as a niche-driven life-history strategy that shapes the eukaryotic host response. Building upon the viral Competitor–Stress-tolerator–Ruderal (CSR) framework originally proposed for phage–bacterium interactions, we propose an extension of this model to the host–microbiome–virome ecosystem, focusing on the host-interactive S-to-R axis to explore how viral ecological strategies may correspond to two divergent immunological profiles supported by emerging molecular, cellular, and clinical data. Within our proposed model, phages exhibiting a “pacifist” profile (candidate S-strategists), co‑evolved with mucosal niches, provide host-outsourced barrier defense and may sequester endotoxins, attenuate reactive oxygen species generation, or reprogram intracellular signaling, such as suppressing nuclear factor-κB (NF-κB) phosphorylation, to silence inflammation and preserve epithelial integrity. Conversely, phages displaying an “aggressor” profile (candidate R-strategists), which act as “ruderals” not only adapted to exploit highly disturbed environments, may also amplify local disturbance by driving localized cytokine cascades. We hypothesize that by triggering pro‑inflammatory bursts, which may contribute to the interleukin (IL) surges observed in successful treatments of refractory infections, these phages outsource matrix clearance to host neutrophils to help breach bacterial biofilms and facilitate systemic dispersion. This framework helps contextualize diverse immunological findings in phage literature, suggesting that pro- and anti-inflammatory host responses may reflect distinct viral life-history trade-offs rather than solely passive reactions to bacterial lysis. While requiring independent CSR trait positioning and further experimental validation, considering these evolutionary strategies may complement current pharmacokinetic perspectives, where “eco‑profiling”, evaluating a phage's eco-evolutionary and immunological profile alongside simple in vitro lysis, could serve as a useful conceptual tool in future translational phage research.