TB Immunology Lecture Notes 2024 PDF

Basics of Tuberculosis Immunology Dr. Bhavna Gordhan Dr. Christopher Ealand Biomedical TB Research Lab School of Pathology University of the Witwatersrand National Health Laboratory Service, South Africa. GEMP lecture...

Basics of Tuberculosis Immunology Dr. Bhavna Gordhan Dr. Christopher Ealand Biomedical TB Research Lab School of Pathology University of the Witwatersrand National Health Laboratory Service, South Africa. GEMP lecture 31st M 15th July 2024 Disease incidence HIV incidence  Mycobacterium tuberculosis Mtb is a small (2-5 µm), aerobic, non-motile bacillus Actinobacteria High lipid content (unique feature contributing to clinical characteristics & pathogenesis) Slow growing (doubles every 16 – 20 hrs) Acid fast Drug susceptible and resistant forms (clinical strains)  What is tuberculosis? Tuberculosis (TB) is an infectious disease caused by the bacteria Mycobacterium tuberculosis (Mtb) Affects the lungs (pulmonary) but can affect other body parts (organs, brain, bones – disseminated or extra-pulmonary) Most infections show NO symptoms (latent) ~1/4 of the world’s population infected Active disease (bloody cough with mucus; fever; night sweats; weight loss; chest pain; shortness of breath) Range of disease (exposure → active disease) Airborne (droplet nuclei; 1-5 bacteria enough to transmit disease)  How is tuberculosis diagnosed? Culture (liquid or solid) Urine testing for LAM (lipoarabinomannan – glycolipid in outer cell wall) Radiographic evidence (Chest X-ray) Immunological detection (IGRA or Mantoux test) Smear microscopy Nucleic acid detection (GeneXpert or Hain LPA)  Treatment of TB First line Isoniazid (INH) Newer drugs Rifampicin Pyrazinamide Ethambutol Streptomycin Second line Kanamycin Rifabutin Thiacetazone Drug-sensitive TB Fluoroquinilones Amikacin Capreomycon Ethionamide/Prothionamide Para aminosalicylic acid Cycloserine Drug-resistant TB  Is there a vaccine for TB? BCG, or bacille Calmette-Guerin, is a (only) vaccine for tuberculosis (TB) disease Older than 100 years!! Mostly Sub-Saharan babies → BCG-vaccinated. BCG is used in many countries with a high prevalence of TB to prevent childhood tuberculous meningitis and miliary disease Variable effectiveness of the vaccine against adult pulmonary TB, and the vaccine’s potential interference with tuberculin skin test reactivity. The protection from the BCG vaccine can last up to 15 years Development of new TB vaccines Development of new TB vaccines  Novel TB vaccine (South African) Shaku, M. T., Um, P. K., Ocius, K. L., Apostolos, A. J., Pires, M. M., Bishai, W. R., & Kana, B. D. (2024). A modified BCG with depletion of enzymes associated with peptidoglycan amidation induces enhanced protection against tuberculosis in mice. eLife, 13, e89157. https://doi.org/10.7554/eLife.89157  What happens when a person is exposed to Mtb? When inhaled, Mtb encounters 1st line of defense: Airway epithelial cells (AECs) & professional phagocytes (neutrophils, monocytes/macrophages & dendritic cells) If 1st line is effective, infection is halted or cleared If not cleared, Mtb reproduces inside phagocytes → initially causing few, if any, clinical symptoms The establishment of infection (active TB) depends on COMPLEX RELATIONSHIP between bacterial and host factors No symptoms but uncleared = LATENT disease  Incipient and Subclinical tuberculosis Historically: scientific research, diagnostics testing & drug treatment have focused on TWO disease states (latent or active) In human TB infection, disease actually exists within a CONTINUOUS SPECTRUM of metabolic bacterial activity & antagonistic immunological responses 5 states: Eliminated Latent Insipient Sub-clinical Active TB By understanding each, different approaches to diagnosis and treatment can be developed  Range of TB disease Mtb has metabolic Radiographic Person has activity to indicate abnormalities or symptoms Categorical Person has viable ongoing or Mtb exposure microbiological suggestive of state of TB Mtb pathogen impending evidence of active, active Mtb progression of viable Mtb disease infection Eliminated TB infection X Latent TB infection X X Insipient TB infection X X X Subclinical TB disease X X X X Active TB disease X X X X X  Bacterial pathogenesis and the immune response At the POPULATION level, genetic heterogeneity (Mtb strains) → influences interactions with host immune system & consequent pathogenesis of TB infection Mtb strains – substantial variation in virulence & immunogenicity → impact propensity to induce disease WGS: Mtb complex strains divided into 7 phylogenetic lineages Lineages 2-4 (genome deletion, TbD1) = modern strains Lineages 1, 5 & 6 = ancient Lineage 7 = intermediate Different Mtb strains induce differential innate & adaptive immune responses Modern – elicit lower level & delayed pro-inflammatory cytokine production from human PBMCs Replicate faster in aerosol-infected mice More pathogenic to mice vs ancient strains Typically BEIJING strains (lineage 2) – multiple human outbreaks (drug resistance) Ancient strains – reduced virulence bacterial burden / differential T-cell responses Given the influence of different Mtb strains on host immune response & disease progression, the state of the pathogen is likely to play a role in shaping the range of disease states associated with TB  Bacterial pathogenesis and the immune response Within an INDIVIDUAL patient, different infection sites within the lung create diverse micro- environments → bacterial phenotypic heterogeneity Variations in cellular compositions & activation levels of immune cells, epithelial cells, and extracellular matrix in granulomas expose Mtb to differences in nutrients, ROS and cytokine profiles, drug penetration Bacterial heterogeneity also influences immune control Primate & human experiments → diverse trajectories for individual granulomas In one study: numbers of viable Mtb within granulomas varied by several orders of magnitude in asymptomatic & symptomatic individuals Tuberculosis Pathogenesis Tuberculosis Pathogenesis Tuberculosis Pathogenesis Tuberculosis Pathogenesis Tuberculosis Pathogenesis Tuberculosis Pathogenesis  Tuberculosis Pathogenesis Low pH Nutrient Deprivation Adapted Drug organisms Treatment Drug tolerant Limited culturability Low LATENT Oxygen Nitrosative Stress  Granuloma physiology Secretion of chemokines and cytokines - Pathogen transmitted via recruitment of Innate immune cells Macrophages inhalation of aerosolized Dendritic cells droplets Monocytes Neutrophils Early defenders to phagocytose pathogen – acidification of phagolysosome Despite pressure from Bacterial burden can ▪ Prevent bacterial host immunity - vary between dissemination pathogen persists granulomas – break ▪ Niches for long term Granuloma Reactivation bacterial survival Latent TB  Adaptation of Mtb in the granuloma Mtb employs many strategies to survive ▪ Transcriptional profiling of intra-phagosomal bacteria show Mtb counters the hostile granuloma environment by ▪ expressing stress adaptive genes. ▪ constitutively expressing genes required for survival Mtb adapted for a lifestyle inside the macrophage  Evasion of host response by Mtb Disruption of phagolysosome integrity and arrest of maturation - Mtb glycolipids prevent accumulation of proteins on phagosomal membranes - Secrete phosphatases and kinases - Lipids can mediate escape from the phagosome and death of the host cell - Secretion system, ESX-1 Mtb has also evolved strategies to evade innate immune responses Strain specific expression of cell envelope component result in differential immune responses ▪ W-Beijing lineage strain HN878 expresses more phenolic glycolipids which are absent in the lab adapted strain, H37RV and other clinical strains ▪ Diminishes production of multiple innate immune cytokines and chemokines – survival Modulate innate immune responses through immune –inhibitory lipid components that compete with immune-activating mycobacterial components for the same receptor. Impair innate immune responses to cell envelope components by enzymatic action ▪ Mtb serine hydrolase, cleaves the multimeric cell wall GroEL2 protein to a secreted monomeric form to mediate attenuated macrophage and dendritic cell responses. A dynamic tug of war between anti-mycobacterial defenses and Mtb Immune evasion.  Innate versus Adaptive Immune Systems Innate immune system – Rapid, Nonspecific defence Key components - leukocytes, dendritic cells, natural killer cells, and plasma proteins — all acting as front-line defenders against pathogens. Not directed against any one pathogen, provides guard against all infection Adaptive Immune System - antigen-specific immune responses. ▪ Helper T cells (CD4+T cells) for adaptive immunity as they activate B cells, macrophages, and cytotoxic T cells - functions to attack the antigen and reduce autoimmunity. ▪ Cytotoxic T cells (CD8+T cells) are responsible for destroying infected or damaged cells, cancer cells, and foreign cells (transplanted organs). Cytotoxic T cell response regulated to dampen the immune response, e.g. organ transplant. ▪ B cells produce antibodies that tag specific pathogens for destruction by components of both the innate and adaptive immune systems. A key role of the adaptive immune system - build a memory bank of antigens for easier recognition and attack of the same antigens in future exposures - faster and stronger due to this memory effect. Cells of the Innate and Adaptive Immune Systems  Macrophages ▪ Macrophages are a type of white blood cell produced by the differentiation of monocytes ▪ Primary function to engulf and digest particles that are detected as antigens professional phagocytes to remove dying, dead or harmful pathogens ▪ Even though phagocytosis is the primary function, also play an essential role in nonspecific defence and adaptive immunity. ▪ Consist of a specialized receptors called Toll-like receptors that recognize products of the bacteria (pathogen) ▪ Receptors also induce inflammatory signalling - express cytokines like IL-6, TNF-α, and IL-2. ▪ Surveillance cells - keep flowing through the blood where they migrate to and circulate within all tissues, patrolling for pathogens or eliminating dead cells and debris. Differentiate into macrophages and dendritic cells - priming of adaptive immunity Combat Mtb infection through the production of reactive nitrogen intermediates (RNI) Delivery of Mtb infected monocyte to pulmonary lymph nodes can coordinate dendritic cells to prime CD4 T cells. Neutrophil Mtb infected murine neutrophils aid in dendritic cell trafficking to the draining lymph nodes to initiate antigen specific CD4 T-cell priming.  Natural killer cells (NK) ▪ Innate lymphocytes with the capacity to secrete IFN-𝛄 - able to perform cytolytic functions ▪ Recognizes components of the Mtb cell wall and stress molecules upregulated on the surface of Mtb infected cells ▪ Mediate direct killing of Mtb infected macrophages ▪ Restrict intracellular bacterial replication via secretion of IL-22 and IFN-𝛄 to increase phagolysosomal fusion of Mtb containing phagosomes ▪ NK cells display characteristics of memory cells – post BCG vaccination – observed expansion of IL-21 dependent NK cells Adaptive Immunity Cell mediated - Cytokine secretion and direct antimicrobial actions of antigen-specific T-cells Dendritic cells (DC) are professional antigen presenting cells that initiate adaptive immunity ▪ Upon Mtb infection, DC mature and migrate to the lung draining lymph nodes to initiate antigen specific T-cell responses Infection of DC with Mtb affects DC maturation resulting in impaired antigen presentation ▪ delay cytokine production and initiation of antigen specific CD4 T-cell responses Interventions or therapies that improve DC functions may increase crosstalk between DCs and antigen specific T-cells  CD4 T-cells and IFN𝛄 response Diminished CD4 cell count in HIV infected individuals – increases risk of Mtb infection ▪ Absolutely necessary to control bacterial replication ▪ CD4-T cells (helper T cells )– assist other blood cells to produce an immune response ▪ CD4-T cells interact with infected macrophages to restrict intracellular Mtb replication ▪ Effectiveness of the CD4 T-cell response depends on the proper homing of antigen -specific CD4 T- cells from the lymphoid tissues to Mtb infected cells in the lung. ▪ Deficiencies related to IFN𝛄 and IL-12 signaling confer fatal susceptibility to mycobacterial infections CD8 T cell response ▪ Cytotoxic T cells that directly kill intracellular Mtb in infected macrophages either by lysis or apoptosis ▪ Secrete cytokines and cytolytic effector molecules that limit bacterial replication ▪ Cells are important components of recall responses to Mtb infection ▪ Important for preventing reactivation of disease CD-8 T-cells play a key role in immunity against Mtb but cannot compensate for CD4 deficiency T-regulatory cells (T-regs) T- regs (suppressor) T cells modulate the immune system, maintain tolerance to self- antigens and prevent autoimmune disease (over inflammation) ▪ Impair anti-mycobacterial T-cell response ▪ Delay the expansion of of CD4 and CD8 T cells - contribute to increased susceptibility to disease  Memory T-cell response Antigen specific memory T-cell responses detected in individuals with LTBI and in TB patients post treatment ▪ CD4 and CD8 T-cells expand rapidly - secrete INF𝛄 – ▪ confer protection during early infection ▪ Unable to confer long-term protection, suggesting that memory T-cells generated after primary Mtb infection have limited capacity to protect from re-infection.  B-cell (lymphocyte) Most intracellular pathogens spread by moving from cell to cell via extracellular fluids. The extracellular spaces are protected by the humoral immune response - antibodies produced by B cells destroy the extracellular microorganisms and prevent the spread of intracellular infections. Activation of B cells and their differentiation into antibody -secreting plasma cells is triggered by antigens and requires helper T-cells B-cells found with T-cells in the lymphocytic cuff of the granuloma B-cells influence the outcome of Mtb infection - moderate the inflammatory response Antibody and cytokine production by B-cells can lead to divergent outcomes Why is M. tuberculosis still a successful pathogen