Evolutionary arms race

Host-Pathogen Coevolution

Introduction to Host-Pathogen Coevolution

  • Definition of coevolution: refers to the process where species influence each other's evolution.

  • Focus of the module: mechanisms by which microbes evade the host's immune response.

  • Importance of understanding coevolution in the context of human immune response and microbial adaptation.

  • Highlight the difference in evolutionary time scales between humans and pathogens, emphasizing the rapidly adapting nature of pathogens.

Evolutionary Dynamics

  • Mutation Rates:

    • Humans: relatively slow mutation rates; variations inherited over generations.

    • Pathogens (e.g., HIV): mutation rates up to a million times higher than humans, allowing rapid adaptation.

  • Implications of high mutation rates for pathogen adaptation to immune responses and antiviral drugs.

Immune Response Overview

  • Arms of the Immune System:

    • Innate Immune Response:

    • Made up of cells (e.g., macrophages) and molecular mechanisms (e.g., epithelial barriers such as skin).

    • Characteristics:

      • Quick, nonspecific response following exposure to pathogens.

      • Examples of innate cells include phagocytes like macrophages, which engulf and digest microbes in phagosomes and lysosomes.

    • Adaptive Immune Response:

    • Composed mainly of B lymphocytes (B cells) and T lymphocytes (T cells).

    • B Cells:

      • Produce antibodies specific to particular pathogens.

    • T Cells:

      • Can directly kill virus-infected cells and remove abnormal cells (e.g., tumor cells).

    • Adapted immune response features:

      • Specific responses and immunological memory, leading to more effective responses upon subsequent exposures.

  • Importance of collaboration between innate and adaptive immune responses.

Mechanisms of Immune Evasion by Pathogens

  • Pathogens' Strategies to Escape Innate Immune Response:

    1. Toxoplasma gondii:

    • Can evade lysosomal fusion, allowing survival within macrophages.

    1. Trypanosoma:

    • Capable of escaping the phagosome, dividing freely within the cell.

    1. Leishmania:

    • Resistant to lysosomal enzymes, can replicate inside the phagosome.

Toll-Like Receptors (TLRs)

  • Definition:

    • TLRs are receptors on immune cells that detect foreign material (common motifs in bacteria and viruses).

  • Key Features:

    • Multi-copy gene family, various TLRs with overlapping but distinct functions.

    • Examples Include:

    • TLR1, TLR2, TLR6: Detect bacterial cell wall components.

    • TLR5: Recognizes flagellin.

    • TLR4: Recognizes lipopolysaccharide (LPS).

    • Intracellular TLRs (TLRs 3, 7, 8, 9): Recognize viral nucleic acids (DNA and RNA).

  • Signaling:

    • Upon detection, TLRs initiate intracellular signaling leading to gene expression of anti-microbial factors (e.g., cytokines).

Genetic Selection of Immune Genes

  • Overview of selective pressure on immune-related genes due to pathogens.

  • Types of Selection:

    • Purifying Selection: Removal of deleterious variations; shown through decreased presence of less fit variants.

    • Positive Selection: Favorable variants increase, leading to selective sweeps.

    • Balancing Selection: Heterozygote advantage, maintaining multiple variants within a population (e.g., sickle cell trait).

Host Pathogen Coevolution Studies

  • Examination of ancient DNA to determine selection pressures from pathogens.

  • Example of research on Yersinia pestis (Black Death) and genetic alterations in populations.

  • Findings:

    • Strong positive selection observed on immune-related genes (e.g., ERAP1 and ERAP2).

    • Changes in allele frequency corresponding to survival advantages during pathogen outbreaks.

Coordination between Immune Responses

  • Details on the interaction of physical barriers and immune cells, including phagocytosis and receptor signaling.

  • Importance of the innate immune system in initiating adaptive responses, leading to coordinated immune actions against pathogens.

  • Clonal proliferation of specific B and T cells during pathogen exposure, leading to effective immune memory.

Antigenic Variation and Immune Escape

  • Antigenic Drift: small mutations leading to reduced antibody recognition, often observed in RNA viruses such as influenza.

  • Antigenic Shift: Major changes in surface proteins leading to pandemics; lack of pre-existing immunity.

  • Examples include seasonal influenza variations and new strains of SARS-CoV-2 showing potential immune escape.

Antimicrobial Resistance

  • Description of how pathogens develop resistance mechanisms against antimicrobial agents.

  • Evolutionary processes that promote drug resistance due to selective pressure from drug use.

  • Discussion on the global burden of drug-resistant infections, particularly in the context of TB, HIV, and malaria.

Global Health Institutions and Initiatives

  • WHO initiatives for addressing antimicrobial resistance on community, agriculture, and industrial levels.

  • Importance of global surveillance of antimicrobial resistance to combat and treat infections effectively in various populations.

  • Examples of Global Impact:

    • Most affected areas and populations by specific pathogens and resistance strains, leading to increased mortality and public health concerns.

    • Highlight strains of multi-drug resistant TB and strategies for management and prevention.

    • Discussions on the need for new drug development in response to emerging resistant pathogens.