Host-pathogen interactions encompass the molecular and cellular events that determine the outcome of infection. These interactions range from pathogen recognition by the innate immune system to pathogen manipulation of host cellular processes for survival and replication.
Pathogen-associated molecular patterns are conserved microbial molecules including lipopolysaccharide, peptidoglycan, flagellin, and nucleic acid motifs. Host pattern recognition receptors including Toll-like receptors, NOD-like receptors, RIG-I-like receptors, and C-type lectin receptors detect these patterns. TLR4 recognizes bacterial LPS, TLR5 recognizes flagellin, and TLR9 recognizes unmethylated CpG DNA.
Following recognition, signaling cascades activate transcription factors including NF-κB, IRF3, and AP-1, leading to pro-inflammatory cytokine and chemokine production. The inflammasome complex activates caspase-1 for processing IL-1β and IL-18. Type I interferons induce the antiviral state through JAK-STAT signaling and interferon-stimulated genes.
Pathogens have evolved sophisticated strategies to subvert host immunity. Bacteria use type III and type IV secretion systems to inject effector proteins into host cells. These effectors manipulate cytoskeletal dynamics, block phagocytosis, inhibit autophagy, and suppress inflammatory signaling. Viral proteins interfere with interferon induction and signaling, inhibit antigen presentation, and block apoptosis.
Intracellular pathogens including Mycobacterium tuberculosis, Salmonella, and Listeria survive within host cells by modifying their phagosomal compartments. M. tuberculosis inhibits phagosome-lysosome fusion. Listeria monocytogenes escapes the phagosome and uses actin-based motility for cell-to-cell spread.
The host immune response involves coordinated action of innate and adaptive immunity. Dendritic cells process antigens and present them to T cells in draining lymph nodes. CD4 T cells differentiate into Th1, Th2, Th17, or Treg subsets based on cytokine environment. CD8 T cells kill infected cells through perforin and granzyme release. B cells produce antibodies that neutralize pathogens and opsonize for phagocytosis.
The study of host-pathogen interactions informs vaccine design, immunotherapy development, and the identification of novel antimicrobial targets. Systems biology approaches integrate transcriptomics, proteomics, and metabolomics to understand infection at a molecular level.
