Antimicrobial resistance is hardest to manage in patients whose own defenses are failing. A new review published in Frontiers in Cellular and Infection Microbiology examines the overlooked half of antibiotic treatment: the host. Written by a team from the China-Japan Union Hospital of Jilin University, the article argues that susceptibility testing and pharmacokinetic/pharmacodynamic (PK/PD) analysis tell clinicians how much drug reaches the bacteria, but they say little about whether injured bacteria are then cleared by the body. In immunocompromised patients, that second step often determines whether treatment succeeds.
The authors organize the problem around distinct functional defects rather than a single label of immunocompromise. Quantitative neutrophil deficiency, as seen after cytotoxic chemotherapy, reduces the sheer number of phagocytic cells available. Qualitative phagocyte dysfunction, found in diabetes, ageing, and inherited disorders, impairs recruitment, bacterial uptake, or intracellular killing even when blood counts look normal. Sepsis can leave monocytes hyporesponsive, measurable by reduced monocyte HLA-DR expression. Pharmacological C5 blockade with eculizumab creates a pathogen-specific vulnerability, dramatically raising the risk of meningococcal infection despite vaccination in most affected patients in long-term pharmacovigilance data. Barrier disruption from mucositis, wounds, or indwelling devices allows protected bacterial communities to establish before immune cells can reach them.
Clinical cohorts illustrate the stakes. Among adult neutropenic patients with hematologic malignancies, researchers identified 43 carbapenem-resistant Enterobacterales bloodstream infections, in which prior CRE culture, selected antibiotic exposures, and glucocorticoid use were associated with bacteremia and active treatment was frequently delayed. In 48 liver transplant recipients with carbapenem-resistant Klebsiella pneumoniae infection, intra-abdominal infection and septic shock were associated with 90-day mortality, while the estimate for effective antibiotic treatment within 48 hours was imprecise and not statistically significant.
The review then turns to how host state changes measured antibiotic pharmacodynamics. Early murine studies showed that granulocyte-mediated bacterial clearance depends on both effector abundance and bacterial burden: killing capacity becomes saturated as bacterial numbers rise, so clearance depends on the relationship between burden and available granulocyte activity rather than a binary neutropenic-versus-non-neutropenic state. Granulocytes markedly enhanced tedizolid activity against Staphylococcus aureus in a thigh model, and in Pseudomonas aeruginosa pneumonia, plazomicin-mediated burden reduction enabled additional granulocyte-mediated killing once bacterial density fell below the saturation range.
A more recent comparison of meropenem in neutropenic, intermediately immunosuppressed, and immunocompetent mice with Klebsiella pneumoniae lung infection showed that immune state changed both bacterial trajectories and model-derived exposure-response relationships. The fraction of the dosing interval during which free concentrations exceeded the MIC correlated most strongly with predicted bacterial counts. At higher exposures, the model predicted a larger drug-attributable reduction relative to untreated controls under neutropenic conditions, while host-mediated clearance contributed more to total control in immunocompetent animals. The authors caution that this study used a meropenem-susceptible laboratory strain, one murine lung model, and categorical immune-state representation, so such findings cannot be translated directly into human dose adjustments.
Antibiotics also change the bacteria that immune cells encounter. Beta-lactam drugs induce different morphologies depending on penicillin-binding-protein selectivity: PBP3-directed activity produces elongated filaments, whereas the carbapenem imipenem produces rounded forms. In an experimental Pseudomonas aeruginosa infection, imipenem-induced rounded cells were more readily phagocytosed than ceftazidime-induced filaments. The amount and inflammatory activity of material released during bacterial killing depend on drug class, concentration, and organism. In severe melioidosis, ceftazidime was associated with significantly greater systemic endotoxin after the first dose than imipenem, though randomized studies in Gram-negative urosepsis and nosocomial pneumonia did not consistently show such differences. Resistance mechanisms themselves reshape recognition: in MRSA, mecA induction increases peptidoglycan degradation and inflammasome-mediated interleukin-1β responses, and sub-minimum-inhibitory ceftazidime exposure in carbapenem-resistant Acinetobacter baumannii induced more membrane vesicles with greater lipopolysaccharide content and inflammatory activity than imipenem.
Host immunity also shapes resistance evolution. In a ciprofloxacin-treated mouse pneumonia model with Acinetobacter baumannii, variants carrying efflux-regulator mutations expanded during antibiotic exposure in neutrophil-depleted mice but were restricted in immunocompetent animals. Notably, treatment failure occurred while most bacteria remained below the clinical ciprofloxacin resistance breakpoint, and continued passage subsequently selected more resistant populations. Whether similar evolutionary trajectories occur in immunocompromised patients remains to be established.
Protected bacterial populations present another barrier. Intracellular Staphylococcus aureus can persist in reversible nondividing states during antibiotic exposure, and in a three-dimensional model, abscess communities surrounded by a fibrin pseudocapsule displayed marked antibiotic tolerance and resisted neutrophil clearance. Biofilms complicate matters further: mature S. aureus biofilms can induce leukocidin-dependent neutrophil extracellular trap formation while remaining resistant to NET-associated clearance. When an infected device or removable focus sustains the burden, source control can restore access for both drugs and immune cells.
Adjunctive strategies vary widely in evidence. G-CSF shortened neutropenia in high-risk febrile neutropenia without a demonstrated mortality benefit; GM-CSF restored monocyte responsiveness in patients selected for low HLA-DR, but a larger trial ended early with insufficient enrollment to resolve clinical outcomes. Prophylactic interferon-gamma reduced serious infections in chronic granulomatous disease, though as prevention rather than treatment of established resistant infection. The granulocyte transfusion RING trial showed no established overall benefit. Bezlotoxumab reduced recurrent C. difficile infection without materially improving initial cure. Suvratoxumab produced a nonsignificant reduction in S. aureus pneumonia, gremubamab did not reduce Pseudomonas pneumonia, and an antibody-antibiotic conjugate for S. aureus bacteremia showed infusion-related reactions and lower exposure in a phase 1b study, with further development not planned. Phage-antibiotic combinations show host dependence: neutrophils substantially enhanced pulmonary phage-mediated bacterial control in experimental models, and in a six-month phage treatment for refractory Mycobacterium abscessus infection, antibody-mediated phage neutralization coincided with loss of benefit.
The authors propose a research framework integrating antibacterial exposure, bacterial phenotype and burden, infection compartment, and host immune function, with paired immune-state models and longitudinal measurement of host and pathogen trajectories. They emphasize that no clinical evidence yet shows that correcting a measured immune deficit improves outcomes of resistant bacterial infection, and that effective antibiotic treatment and source control when indicated remain the foundations of care. The framework itself has not undergone clinical validation, and its premature application to individual dosing decisions could lead to inappropriate treatment.
Subject of Research: How innate immune defects interact with antibiotic activity in resistant bacterial infections among immunocompromised patients.
Article Title: Innate immunity and antibiotic activity in resistant bacterial infections in immunocompromised hosts
Article References: Sun, Y., Guo, Y., Cui, H., Jiang, Y., & Yu, Q. (2026). Innate immunity and antibiotic activity in resistant bacterial infections in immunocompromised hosts. Frontiers in Cellular and Infection Microbiology, 16, Article 2002377. https://doi.org/10.3389/fcimb.2026.2002377
Image Credits: AI Generated
DOI: 10.3389/fcimb.2026.2002377
Keywords: antimicrobial resistance, innate immunity, immunocompromised hosts, neutrophils, complement, pharmacokinetics, adjunctive therapy, bacterial persistence, phage therapy, monoclonal antibodies, Innate, immunity
News Source: Kristina Jarvis. (October 29, 2026). When Immune Defenses Fail, Antibiotics Fight Alone Against Resistant Bacteria. Scienmag.
