Delayed-type hypersensitivity (DTH) is a unique type of cell-mediated immunity. The name originated from the skin test used in the diagnosis of tuberculosis and denotes cellular infiltrates causing induration and erythema at the skin test site within 24 to 72 hours. DTH was coined to describe the reaction to the tuberculosis skin test and to differentiate between delayed cellular skin test results and antibody-mediated immediate skin test results. The term has been expanded to include cell-mediated reactions to bacteria or fungi infecting lungs and skin responses to chemicals and plants. In the lung, cellular responses to tuberculosis, histoplasmosis, and coccidioidomycosis are considered DTH reactions. Skin reactions to nickel and poison ivy also are DTH reactions.

The principal effectors of DTH reactions are CD4Th1 lymphocytes, monocytes or macrophages, CD8Tc1 cells, and natural killer (NK) cells. The histopathology of DTH lesions varies with the nature of the antigen, the type of effector cells, and the anatomic location.

Delayed-Type Hypersensitivity Responses in the Lung

Tuberculosis

The family Mycobacteriaceae contains the Mycobacterium genus. Within the genus are six different human pathogens (Table 19-1). (1) M. tuberculosis causes most of the 9.1 million cases of pulmonary tuberculosis in the world. (2) M. leprae, which is also known as Hansen’s bacillus, is commonly found in tropical countries and is the etiological agent of leprosy. (3&4) Mycobacterium avium and M. intracellulare are found in soil, water systems, birds, and other domestic animals. Because the two species are difficult to identify in the laboratory, they are designated as the Mycobacterium avium complex (MAC). Patients with acquired immunodeficiency syndrome (AIDS) and therapeutically immunosuppressed individuals are frequently infected by MAC organisms. (5) M. bovis is a cattle-adapted species of tuberculosis that is also found in wild and domestic animals, but it causes less than 1% of all tuberculosis infections. Human infection is caused by inhalation of droplets containing live organisms or by ingestion of unpasteurized milk. Ingestion leads to extrapulmonary tuberculosis in lymph nodes and skeletal bones. (6) Mycobacterium africanum is commonly found in West Africa, where it accounts for 25% of tuberculosis cases.

Table 19-1

Species of Tuberculosis Infecting Humans

Pathogen	Infection/Transmission
Mycobacterium tuberculosis	Human tuberculosis. Transmitted by inhalation route
Mycobacterium leprae	Hansen’s disease, or leprosy. Transmitted by inhalation route
Mycobacterium avium-intracellulare	Group of closely related opportunistic pathogens that infect immunosuppressed patients with human immunodeficiency virus (HIV). Transmitted by inhalation and oral routes
Mycobacterium bovis	Tuberculosis in cattle. Can be transmitted to humans via infected milk or aerosols
Mycobacterium africanum	Commonly found in West Africa. Spread by the airborne route

Tuberculosis: Virulence Factors

In the lung, survival of the tuberculosis bacterium is determined, in part, by cell-wall mycolic acids and a lipoarabinomannan known as the cord factor. Mycolic acids, which are 50% of the dry weight of the cell wall (Figure 19-1), form a hydrophobic barrier that prevents the entry of drugs, disinfectants, and other harsh chemicals. Cord factor inhibits the interferon (IFN)–induced activation of macrophages and stimulates the production of tumor necrosis factor alpha (TNF-α).

Figure 19-1 Mycobacterial cell wall. The mycobacterial cell envelope is rich in waxes and lipids, especially mycolic acids, which are covalently linked to arabinogalactans. The unique long-chain mycolic acid trehalose 6,6’-dimycolate (C₆₀–C₉₀) is responsible for the “cording” seen with virulent strains, giving colonies a serpentine morphology. Mycolic acids form an asymmetric lipid bilayer with shorter-chain glycerophospholipids. The cell envelope also contains peptidoglycan and has a complex glycolipid structure forming the outermost layer, which sits atop the peptidoglycan and lipid bilayer of the cytoplasmic membrane. Other important components include lipoarabinomannan and arabinogalactan. (From Actor JK: Elsevier’s integrated immunology and microbiology, Philadelphia, 2007, Mosby.)

Laboratory Identification of Mycobacterium Species

Laboratory identification of mycobacteria is difficult because conventional stains do not penetrate the cell wall. However, mycobacteria can be identified by the Ziehl Neelsen stain, which uses a combination of heat and carbolfuchsin to drive the stain into the cell wall. Only the Mycobacterium and Nocardia species retain the dye following harsh treatment with an acid–alcohol mixture. Thus, these species are designated acid-fast organisms.

Pathophysiology of Tuberculosis

Exposure to tuberculosis occurs by inhalation of droplets containing live organisms. Because of their small size, the organisms are usually deposited in the distal areas of the lung. In the first stage of the disease, mycobacteria take up residence in phagosomes within alveolar macrophages and prevent intracellular killing by inhibiting maturation of the phagosome and fusion with endosomes and lysosomes which contain antimicrobial proteins and enzymes. Rapid logarithmic multiplication of mycobacteria in macrophages eventually causes the death of the cells and creates a small necrotic area in the lung. Free mycobacteria are ingested by circulating monocytes and transported to the mediastinal lymph node. Antigen stimulation in the lymph node generates CD4Th1, CD8, and long-lived memory cells.

In the second stage, which occurs 3 weeks after infection, CD4Th1 cells migrate from the lymph node to the infection site and undergo rapid proliferation. CD4Th1 and CD8 cells secrete TNF-α and IFN-γ. TNF-α is responsible for most of the lung damage noted in mycobacteria infections. IFN-γ– stimulated macrophages produce nitric oxide by oxidation of L-citrulline. Additional interactions between nitric oxide and singlet oxygen form peroxynitrite (ONOO–), which is microcidal. The role of CD8 cells in tuberculosis is unclear. They may contribute to the lesion by producing IFN-γ, by directly killing infected cells, or by both mechanisms (Figure 19-2).

Figure 19-2 The central area of caseous necrosis in which much of the cellular structure is destroyed is characteristic of tuberculosis in the lung. Apart from this necrosis, the histology is typical of chronic T cell–dependent “tuberculoid” granulomas. The lesion is surrounded by a ring of epithelioid cells and mononuclear cells. Multinucleate giant cells, believed to be derived from the fusion of epithelioid cells, are also present (*left*: ×170). Giant cells *(G)* are illustrated at a higher magnification *(right).* (H&E stain.) (In Roitt I, Brostoff J, Male D, et al: Immunology, ed 7, Philadelphia, 2006, Mosby. Courtesy of Dr. G. Boyd.)

In the third phase of the disease, macrophage-derived reactive oxygen species cause necrosis of lung tissue. The production of chemotactic factors causes an influx of activated monocytes into the area. In an attempt to destroy the bacteria, activated monocytes surround the caseous center and release additional cytotoxic molecules. Some monocytes become flattened and are known as epithelioid cells (Figure 19-3). Fusion of macrophage membranes often creates giant cells with multiple nuclei.

Figure 19-3 The epithelioid cell is the characteristic cell of granulomatous hypersensitivity. The extent of the endoplasmic reticulum *(E)* is compared. 1, In the epithelioid cell (×4800). 2, In a tissue macrophage (×4800). (C, Collagen; L, lysosome; M, mitochondria; N, nucleus; U, nucleolus.) (In Roitt I, Brostoff J, Male D, et al: Immunology, ed 7, Philadelphia, 2006, Mosby. Courtesy of M.J. Spencer.)

In a final attempt to contain the infection, fibroblasts are called into the area and produce collagen, which encapsulates the inflammatory cells surrounding the lesion. The accumulation of macrophages, lymphocytes, and fibroblasts is called a granuloma. Constant production of TNF and IFN-γ is required for the maintenance of the granuloma; if the granuloma is not maintained, it becomes unstable, liquefies, and creates a permissible environment for bacterial growth. As a consequence of rapid bacterial growth, the infection erodes into blood vessels and small airways. Dissemination in blood allows the organism to establish secondary infections in other organs and bone. This form of the disease is known as miliary tuberculosis. Mycobacteria in the airways also rapidly proliferate in the highly aerobic environment and are expelled from the lung by coughing or sneezing.

Genetics and Mycobacterium Infections

The role of host genetics in Mycobacterium infection was suggested by an accidental administration of virulent M. tuberculosis to 249 babies in Lubeck, Germany, in 1926. Although 76 babies died of the disease, 123 had lung lesions that healed, and another 50 babies had no signs of infection. This suggested that genetics influences susceptibility to tuberculosis. Subsequent studies indicated that persons at risk for active tuberculosis have dysfunctional receptors for IFN-γ or vitamin D. Some individuals also have a defect in the production of natural resistance–associated macrophage proteins (NRAMPs).