Immunity to Toxigenic Bacteria

In disease caused by toxigenic bacteria such as clostridia or Bacillus anthracis, the immune response must not only eliminate the invading bacteria but also neutralize their toxins. Destruction of the bacteria, however, may be difficult if they are embedded in a mass of necrotic tissue, and toxin neutralization is a priority. Neutralization occurs when antibody prevents the toxin from binding to its receptors on a target cell. The neutralization process therefore involves competition between receptors and antibodies for the toxin molecule. Once the toxin has bound to its receptors, antibodies are relatively ineffective in reversing this combination.

Immunity to Invasive Bacteria

Protection against invasive bacteria is usually mediated by antibodies directed against their surface antigens. Efficient phagocytosis requires that the bacteria be coated with opsonins that can be recognized by phagocytic cells. These opsonins include antibodies and C3b in addition to the innate opsonins such as MBL. Antibodies not only are effective opsonins in their own right but also increase the binding of C3b by activating the classical complement pathway. Antibodies directed against capsular (K) antigens may neutralize the antiphagocytic properties of bacterial capsules, thus permitting their destruction by phagocytic cells. In bacteria lacking capsules, antibodies directed against O antigens act as opsonins. Protection also results when antibodies are produced against the Escherichia coli pilus antigens F4 (K88) and F5 (K99). The antibodies may interfere with the expression of pili. Once the adherence pili are suppressed, these strains of E. coli cannot bind to the intestinal wall and thus are no longer pathogenic.

The importance of bacterial capsules in immunity is seen in anthrax. B. anthracis organisms possess both a capsule and an exotoxin. Antitoxic immunity is protective but slow to develop. In addition, toxin production tends to be prolonged since the organism is encapsulated and phagocytic cells have difficulty eliminating it. As a result, death is usually inevitable in unvaccinated animals. The vaccine commonly employed against animal anthrax contains an unencapsulated but toxigenic strain of B. anthracis. Given in the form of spores that can germinate, the unencapsulated bacteria are eliminated by phagocytic cells before dangerous amounts of toxin are synthesized but not before antitoxic immunity is established.

Molecule for molecule, immunoglobulin M (IgM) is about 500 to 1000 times more efficient than IgG in opsonization and about 100 times more potent than IgG in sensitizing bacteria for complement-mediated lysis. During a primary immune response, therefore, the quantitative deficiency of the IgM response is compensated for by its quality, ensuring early and efficient protection.

The traditional view of antibodies has been that they alone could not kill microorganisms, but simply could mark microorganisms for destruction. That view is now known to be incorrect. Many antibodies have direct antimicrobial activities. Antibodies against E. coli may be bacteriostatic since they interfere with production of the iron-binding protein enterochelin and thus prevent bacterial iron scavenging. IgM and IgG antibodies against Borrelia burgdorferi damage surface proteins on the bacteria and are bactericidal in the absence of complement. There is also evidence that antibodies are able to generate oxidants and may kill bacteria directly.

Immunity to Intracellular Bacteria

As discussed in Chapter 18, some bacteria such as Brucella abortus, Mycobacterium tuberculosis, Campylobacter jejuni, R. equi, Listeria monocytogenes, Corynebacterium pseudotuberculosis, C. burnetii, and some serotypes of S. enterica can grow readily inside macrophages. In addition, L. monocytogenes can travel from cell to cell without exposure to the extracellular fluid through cytoskeletal membrane protrusions.

Autophagy, as described in Chapter 4, is also a key component of the destruction of intracellular bacteria. The same cellular machinery used to destroy unwanted organelles can be employed to eliminate intracellular organisms. Autophagy (or more correctly, xenophagy) may also play a key role in delivering microbial antigens to the appropriate major histocompatibility complex (MHC) molecules.

Protection against intracellular bacteria is mediated by macrophages activated through the M1 pathway. Classically activated M1 macrophages are responsive to inflammatory cytokines and microbial products (Chapter 5). Although macrophages from unimmunized animals cannot usually destroy these bacteria, this ability is acquired about 10 days after onset of infection once the macrophages are activated (Chapter 18). IFN-γ, especially in association with TNF-α, greatly enhances the production of cytokines such as TNF-α, IL-6, IL-1β and IL-12, enzymes such as indoleamine 2,3-dioxygenase (IDO) and nitric oxide synthase 2 (NOS2), and the release of reactive oxygen and nitrogen intermediates. M1 polarization has been shown to be important in resistance to L. monocytogenes, S. enterica Typhi and Typhimurium, mycobacteria, and chlamydia. For example, IFN-γ and TNF-α produced by primed T cells generate M1 macrophages, acidify their phagosomes, and kill mycobacteria. Uncontrolled M1 activation by organisms such as streptococci and E. coli, however, can contribute to pathology by inducing, for example, sepsis, tissue damage, and organ failure. The response of these activated macrophages tends to be nonspecific, particularly in listerial infections, and M1 macrophages are able to destroy many normally resistant bacteria. Thus an animal recovering from an infection with L. monocytogenes develops increased resistance to infection by M. tuberculosis. The development of M1 macrophages often coincides with the appearance of delayed (type IV) hypersensitivity responses to intradermally administered antigen (Chapter 31).

Both CD4+ and CD8+ cells are also involved in immunity to Listeria. CD8+ cytotoxic T cells lyse listeria– or mycobacteria-infected cells and complement the Th1 cells that activate the macrophages. R. equi–infected macrophages are recognized and killed by CD8+ T cells in a MHC class I unrestricted manner.

It has been observed that protective immunity against intracellular bacteria cannot be induced by vaccines containing killed bacteria. Only vaccines containing living bacteria are protective. This is because of the differential stimulation of helper T cell populations by live and dead bacteria. Infection of mice with live B. abortus stimulates Th1 cells to secrete IFN-γ. Conversely, immunization of these mice with Brucella protein extracts induces Th2 cells to secrete IL-4. Likewise, live but not dead L. monocytogenes or B. abortus organisms induce macrophage secretion of TNF-α. Killed Brucella organisms stimulate IL-1 production to a greater extent than live bacteria. Resistance to these intracellular bacteria is generally short lived, persisting for only as long as viable bacteria remain in the body. (Tuberculosis is an exception, in which case memory is prolonged.)

If, in a bacterial disease, it is observed that dead vaccines do not give good protection, that serum cannot confer protection, that antibody levels do not relate to resistance, and that delayed hypersensitivity reactions can be elicited to the bacterial antigens, the possibility that cell-mediated immunity may play an important role in resistance to the causative organism should be considered, and the use of vaccines containing living bacteria should be contemplated.

Modification of Bacterial Disease by Immune Responses

The immune response clearly influences the course and severity of an infection. At best, it will result in a cure. In the absence of a cure, however, the infection may be profoundly modified. Much depends on whether a cell-mediated or antibody response is generated. Thus the type of helper T cells induced during infection may affect the course of disease. As described in Chapter 18, cell-mediated responses are required to control intracellular bacteria since only activated macrophages can prevent their growth. Macrophage activation requires that Th1 cells produce IFN-γ. Once activated, these M1 cells can localize or cure these infections. If an animal mounts an inappropriate Th2 response, cell-mediated immunity fails to develop, M2 macrophages are generated, and chronic progressive disease may result. This is readily seen in mycobacterial diseases. For example, in humans, leprosy occurs in two distinct forms called tuberculoid and lepromatous leprosy. Tuberculoid (or paucibacillary [PB]) leprosy is characterized by an intense cell-mediated immune response dominated by Th1 cells and M1 macrophages. Lesions of this form of the disease contain very few organisms. Lepromatous (or multibacillary [MB]) leprosy, in contrast, is characterized by very high antibody levels and poor cell-mediated responses. Humans with lepromatous leprosy employ Th2 cells secreting IL-4 and IL-10. The IL-10 reduces the production of IL-12, which in turn decreases IFN-γ secretion by Th1 cells and generates M2 macrophages. This reduces the patient’s ability to control Mycobacterium leprae, and their lesions contain enormous numbers of bacteria. The prognosis of lepromatous leprosy is much poorer than for tuberculoid leprosy.

A similar diversity of lesions is seen in Johne’s disease of sheep. Some animals develop MB disease, in which their intestinal lesions contain enormous numbers of bacteria (Figure 25-3) and little histological evidence of a cell-mediated response. Their granulomas tend to lack organization, with large numbers of bacteria-laden macrophages intermixed with lymphocytes. In contrast, other sheep may develop PB disease, in which the lesions contain very few bacteria but large numbers of lymphocytes. These are organized nodular lesions with epithelioid cells and multinucleated giant cells at the center surrounded by fibrous connective tissue. The two forms of the disease are associated with differential expression of cytokine and chemokine receptors. Thus animals with the PB disease have increased numbers of CD25+ T cells that produce more IL-2 and much more IFN-γ than sheep with the MB form of the disease (Figure 25-4). In contrast, sheep with the MB disease have higher antibody levels and a lack of cellular immune responses. It is likely, therefore, that sheep with PB lesions mount an immune response in which Th1 cells predominate, whereas those with MB disease use Th2 cells.