Defense of the Lungs Against Infection
1. Learning Objectives:
- Be able to explain how the mucociliary apparatus helps protect the respiratory tract from pathogenic organisms. Describe the changes during viral or Mycoplasmal infection and how they could result in less efficient function, predisposing to bacterial secondary infection.
- Describe BALT (Bronchus-Associated Lymphoid Tissue)—its location, function, and size changes with respiratory infection.
- Be able to explain the unique features of both synthesis and function of secretory IgA.
- Describe 4 respiratory pathogens and explain how each overcomes normal host defense mechanisms to colonize the respiratory tract.
2. Respiratory System Defense Mechanisms
2.1. Constant exposure of respiratory system to microbes
- ambient environment
- aerosolized from other individuals
- oropharyngeal flora
- Adapted for minimal thickness of air/blood barrier
- Lower airways remain sterile unless:
- overwhelming inoculum
- unusually virulent organism
- initial exposure to a new species
2.2. Non-immune mechanisms
- particulate deposition: size determines site of deposition
- mucociliary clearance
- airway and alveolar macrophage phagocytosis: requires opsonization for gram negative bacteria
- PMN phagocytosis: recruited from circulating pool
- lactoferrin: interferes with microbial iron metabolism
- lysozyme: general antimicrobial activity
- interferon: can enhance non-immune resistance
- surfactant: non-immune opsonization of gram positive bacteria
- fibronectin fragments: non-immune opsonization
2.3. The Mucociliary apparatus
- Functions as a mechanical, chemical and biological barrier in the airways
- Mucus viscosity is increased (which slows clearance) by increased serum proteins, inc. DNA (inflammatory cells) and poor mucous hydration
- All of the following decrease mucociliary function: atropine administration, physical airway injury (endotracheal tube cuff inflation, airway suction, bronchoscopy, prolonged coughing), chemical injury (cigarette smoke, pollutants—incl. ammonia) , bacterial attachment to airway cells, soluble bacterial products, viral infection, inflammatory mediators, and allergen challenge.
2.4. Factors that impair normal host respiratory defenses
- Viral infection
- Viral pathogens often infect and injure airway or alveolar epithelial cells. The resulting cytopathic changes breach the integrity of the epithelial barrier, exposing deeper tissues to inhaled pathogens, allergens, or chemical toxins.
- Viral infection of epithelial cells can alter mucociliary function or the amount and rheologic characteristics of the mucous layer. Either will impair mucociliary clearance, allowing bacterial or mycoplasmal pathogens prolonged access to the epithelial surface.
- Examples of very effective respiratory viral pathogens that often predispose to secondary viral infection are:
- Canine: canine parainfluenza virus, canine adenovirus type-2. canine distemper virus
- Feline: feline herpesvirus (FVR) and calicivirus
- Bovine: Parainfluenza -3, IBR, Bovine respiratory syncytial virus
- Equine: Equine Herpesvirus I (Equine rhinopneumonitis), Equine type A Influenzavirus
- Porcine: porcine circovirus type 2, porcine respiratory corona virus, swine influenza virus (H1N1 and H3N2)
- Transportation, chilling, overheating, dehydration, and social stress cause release of endogenous cortiosteroids and reduced efficiency of respiratory defense mechanisms;
- Environmental irritants
- Ammonia from soiled bedding accumulates in many different housing situations and reduces the effectiveness of mucociliary clearance.
3. Immunologic Defense of the Respiratory System
3.1. Bronchus-associated lymphoid tissue (BALT)
- Three Types:
- lymphoid nodules (encapsulated) associated with bronchi, especially at bifurcations (These have high numbers of IgA-secreting precursor lymphocytes)
- local aggregates of lymphoid and non-lymphoid cells within the connective tissue
- lymphocytes (mainly T helper) in airway walls and alveolar lining layer
- Present in mammals, birds, reptiles. There is species variation in relative amounts. BALT is minimal in human lung.
- Develops after birth at sites of particle deposition; minimal in germ-free animals, and expands with antigen exposure.
- Non-ciliated lymphoepithelium overlies BALT that allows passage of soluble and particulate antigenic material from airways to underlying lymphoid follicles.
- specialized capillary endothelial cells facilitate transit of lymphocytes from blood into BALT (mediated by surface molecules called vascular addressins and lymphocyte homing receptors)
- Antigen-specific antibody-producing cells are recruited to the lungs after lung challenge with antigen.
- Common mucosal immune system:
- lymphocyte traffic between mucosal surfaces and central lymphoid tissue
- antigen presentation at single site leads to general mucosal resistance
3.2. Respiratory immunoglobulins
- Secretory IgA:
- Structure: Ig molecule + secretory piece
- Synthesis: IgA-committed plasma cells in BALT and receptor-mediated uptake and transport through epithelial cells
- Functions: Virus and toxin neutralization, prevention of bacterial and viral attachment to epithelial surface, bacterial agglutination to facilitate phagocytosis
- Source: Transudation from plasma and local synthesis by l plasma cells
- Function: Opsonization, complement fixation, ab-dependent cytotoxicity (ADCC)
- IgE (submucosal mast cells)
- IgM (very small amounts)
4. Infection and the Respiratory System
4.1. Normal mucosal flora
- Protective effect of normal flora: prevents colonization with more pathogenic strains
- Changes in normal flora with stress and infection
4.2. Effects of viral infection
- Loss of epithelial integrity: Epitheliotropic viruses.
- Loss of normal mucociliary clearance:
- epithelial injury
- changes in mucus character
- Compromise of phagocytic function: Adhesion, phagocytosis, intracellular microbe killing.
- Immunologic response: May be aimed at phagocytes expressing viral antigens.
5. Examples of Successful Bacterial Respiratory Pathogens
5.1. Canine respiratory bordetellosis/infectious tracheobronchitis (ITB)
Bordetella bronchiseptica (gram neg. coccobacillus) is the principal etiologic agent
- pathogenic without previous or concurrent viral infection, but often assoc. with Canine parainfluenza virus and canine adenovirus-2
- host range: dogs, pigs, cats, laboratory animals. Related to B. pertussis (human whooping cough).
- finbriae and associated adhesion proteins mediate epithelial adherence via specific receptors
- Virulence factors
- exotoxins inhibit phagocytosis by PMN and macrophages
- dermonecrotic toxin suppresses humoral and cell-mediated immune response
- tracheal cytotoxin facilitates colonization and loss of ciliary activity
- Importance of local mucosal antibody production
- whole cell bacterin provides systemic (IgG) immunity, which protects from disease, while intranasal vaccine stimulates IgA and is protective against colonization. Vaccine development is currently aimed at adhesion proteins.
5.2. Equine rhodococcus equi infection
- Gram-positive facultative intracellular pathogen that causes chronic severe pyogranulomatous bronchopneumonia in foals, especially on breeding farms.
- Virulent strains have surface virulence-assoc. proteins (VAPs), and specific antibodies may be protective, but host immune status is difficult to correlate with infection.
- R equi invades equine macrophages and prevents phagosome-lysosome fusion and the respiratory burst. Antibody opsonization enhances macrophage microbicidal ability.
- PCR assay on bronchoalveolar lavage fluid samples can be used for rapid confirmation of the diagnosis.
5.3. Bovine pneumonic pasturellosis (Bovine Resp. Disease Complex/BRDC)
- Mannheimia (Pasturella) hemolytica serovar 1: Reservoir is tonsil in healthy cattle, and rapid multiplication occurs in the nasopharynx with stress. Inhalation of organisms results in pulmonary infection and fibrinopurulent pneumonia.
- Adherence to epithelial cells prevents mucociliary clearance and promotes bacterial replication.
- Leukotoxin (an exotoxin) from log phase cells binds integrins on bovine leukocytes and induces apoptosis. It is cytotoxic to both bovine macrophages and neutrophils.
- M. hemolytica endotoxin enhances PMN acherence to vascular endothelium, facilitating diapedesis and potential neutrophil-mediated injury to the epithelial barrier.
- Consequences for vaccination: whole cell bacterin administration caused a higher incidence of pneumonia and more severe pulmonary lesions. Live organism vaccine results in anti-leukotoxin antibodies and less severe lesions.
5.4. Mycoplasma sp.
- The organism: Class Mollicutes with 2 genera, Mycoplasma and Ureoplasma that have animal pathogens. Mycoplasma differ from bacteria in lack of a cell wall—they have only a cell membrane and have a need to obtain sterols from host cell membranes. They often attach to cilia and remain on the airway luminal surface without penetrating into the parenchyma.
- Mycoplasma have the smallest genome of any free living organism. Contact between mycoplasma and host cells allows accumulation of metabolic end produces (peroxides and oxygen radicals) that damage host cell membranes.
- Pathophysiology of lung injury: Adhesion of mycoplasma to respiratory epithelial cilia causes loss or clumping of cilia and compromise of the mucociliary escalator, resulting in accumulation of mucus and inflammatory exudate with obliteration of the airway lumen and local atelectasis.
- Stimulation of goblet cell activity augments accumulation of mucus, and release of inflammatory mediators by host cells results in bronchoconstriction.
- Persistence of the organisms at the mucosal surface causes prolonged immunogenic stimulation. BALT hyperplasia is characteristic of mycoplasmal infection. Massive lymphoid hyperplasia causes local compression and atelectasis.
- Mycoplasma evade the immune response by impairing lymphocyte function (humoral) and suppressing cell mediated responses such as phagocytosis. Thus Mycoplasma infection is often chronic and low grade, but predisposes the host to secondary bacterial infection.
- Some classic mycoplasmal respiratory “syndromes”:
|Host||Mycoplasma||Chronic Respiratory Disease|
|Rodent||M. pulmonis||murine respiratory mucoplasmosis|
|Swine||M. hyopneumoniae||enzootic porcine pneumonia/PRDC|
|Poultry||M. gallisepticum||avian mycoplasmosis|
|Cattle||M. mycoides/mycoides SC||contageous bovine pleuropneumonia /CBPP|
|M. bovis||Enzootic pneumonia of calves|
|Goats||M. mycoides/mycoides LC or capri||Pleuropneumonia/arthrtis|
- M. gallisepticum infection causes significant economic losses in poultry in the US. An eradication program is currently in progress.
- Porcine respiratory disease complex (PRDC) is a multifactorial respiratory syndrome that includes M. hyopneumoniae along with viral agents. Porcine enzootic pneumonia generally referrs to low grade respiratory infection with M. hyopneumoniae alone.
- Goats are infected with M. mycoides subspecies mycoides LC (now called capri) that causes pleuropneumonia and arthritis in goats and occasionally sheep in the US. M. capricolum causes a more severe contagious caprine pleuropneumonia (CCPP) that is a major threat to goat farming in Africa and Asia.
- Small animals and horses appear to have few significant host-adapted mycoplasmal pathogens. Mycoplasma sp. are part of the normal upper respiratory flora of normal dogs and cats and are sometimes cultured from cases of pneumonia. Whether their role is primary or secondary is uncertain.
- M. bovis is considered an emerging respiratory pathogen, especially in calves. It is economically quite important in Europe and South America.
- Contagious bovine pleuropneumonia (CBPP) is a serious contagious bovine respiratory pathogen that was eradicated in the US in 1892. It continues to be a significant disease factor in Africa, southern Europe, Asia and the Middle East. It is considered a possible bioterrorism agent.
- M mycoides subsp. mycoides SC is the agent, and causes a purulent fibrinous peluropneumonia. This was the first mycoplasma species isolated.
M. pulmonis causes murine respiratory mycoplasmosis in rats and mice. It is highly contagious, especially in intensive housing situations. Most contemporary rodent facilities are careful to introduce only disease-free animals and maintain colonies free of this pathogen.
- Histopathologic Changes and Clinical Signs:
- Nasal inflammation and pharangitis; associated clinically with abnormal upper respiratory sounds (stertor/stridor).
- Otitis media; associated clinically with vestibular irritation and twirling or spinning in rodents
- Pulmonary lesions are often hyperplasia of bronchus associated lymphoid tissue (BALT), bronchiectasis, and consolidation of lung tissue.
- Airway epithelial hyperplasia and squamous metaplasia occur with chronic infection.
- Histopathologic Changes and Clinical Signs:
- Chaffin MK et al. Foal-related risk factors associated with development of Rhodococcus equi pneumonia on farms with endemic infection. J Am Vet Med Assoc 223;1791-1799, 2003
- Clarke JM et al. Development of an ex vivo model to study adherence of Mannheimia haemolytica serovar 1 to mucosal tissues of the respiratory tract of cattle. Am J Vet Res 62; 805-811, 2001.
- Clinkenbeard KD et al. Role of Pasteurella haemolytica leukotoxin in virulence and immunity in shipping fever pneumonia. Compend Cont Ed Pract Vet 14:124901260, 1992.
- Cohen ND et al. control and prevention of Rhodococcus equi pneumonia in foals. Compend Cont Ed Pract Vet 22:1062-1070, 2000.
- Hooper-McGrevy KE et al. Evaluation of equine immunoglobulin specific for Rhodococcus equi virulence-associated proteins A and lC for use in protecting foals against R. equi-induced pneumonia. Am J Vet Res 62; 1307-1313, 2001.
- Keil D, Benwick B. Role of Bordatella bronchiseptica in infectious tracheobronchitis in dogs. JAMA 212: 200-207, 1998.
- McClenahan DJ et al. In vitro evaluation of the role of platelet-activating factor and interleukin-8 in Mannheimia haemolytica-induced bovine pulmonary endothelial cell injury. Am J Vet Res 63:394-401, 2002.
- Nicholas RA. Contagious caprine pleuropneumonia. IN: Recent Advances in Goat Diseases, M. Tempesta (ed). 2002.
- Radi ZA et al. Effects of the synthetic selectin inhibitor TBC1269 on tissue damage during acute Mannheimia haemolyticia-induced pneumonia in neonatal calves. Am J Ret Res 62: 17-22, 2001.
- Sarradell J et al. A morphologic and immunohistochemical study of the bronchus-associated lymphoid tissue of pigs naturally infected with Mycoplasma hyopneumoniae. Vet Pathol. 40;395-404, 2003.
- Vivrette, SL et al. Clinical application of a polymerase chain reaction assay in the diagnosis of pneumonia caused by Rhodococcus equi in a horse. J Am Vet Med Assoc 217;1348-1350, 2000.
- Wanner A et al. Mucociliary clearance in the airways. Am J Respir Crit Care Med 154:1868-1902, 1996.