Tularemia is caused by the facultative intracellular bacterium Francisella tularensis. Attenuated live vaccines, such as F. tularensis LVS (live vaccine strain), afford good--although not complete--protection; how to judge the degree of this protection has long been a problem. Both natural infection and vaccination result in immunospecific and long-lasting humoral and cell-mediated immunity. The latter is the crucial protective mechanism, whereas the humoral response protects only against strains of reduced virulence, such as those used in the vaccines. Immune serum has been used to screen for structures of F. tularensis with the ability to induce a protective immune response. This immune serum is, however, primarily directed toward antigens different from those involved in cell-mediated immunity. Serum antibodies from primed individuals recognize carbohydrate capsule antigens of F. tularensis, whereas T lymphocytes recognize membrane polypeptides of the organism. The preparation of membrane polypeptides from F. tularensis is now facilitated by the availability of a capsule-deficient mutant of F. tularensis LVS. In vitro, several membrane polypeptides of the mutant stimulate T lymphocytes from vaccinees and from naturally infected individuals. Further studies of the mechanisms of induction of protective immunity should focus on these membrane polypeptides.

Nature of Protective Immunity to Francisella tularensis

Studies of immune responses to Francisella have been conducted for well over 50 years. Here, the basic parameters of innate and adaptive immune responses to Francisella are reviewed, with an emphasis on those that may contribute directly to protection against infection. Although older literature provides a wealth of information on human immune responses to infection and vaccination, most recent information has been derived largely from studies in animals and using animal cells, particularly mice. In experimental animals, activation of macrophages, a major and probably preferred host cell for Francisella, appears central to control of infection. Thus, in animal models and in vitro studies using mouse macrophages, cytokines such as IFN-gamma and TNF-alpha, derived first from both nonspecific cells such as natural killer cells and later from Francisella-specific T cells, collaborate to effect intracellular killing. In mice, these intracellular killing mechanisms include reactive nitrogen and oxygen species, but killing mechanisms remain to be identified in humans. Ultimately both CD4(+) and CD8(+) T cells develop into Francisella-specific memory cells and are important for control of primary Francisella infection or vaccination-induced protection. The effector mechanisms invoked by either CD4(+) or CD8(+) T cells, beyond production of IFN-gamma and TNF-alpha, are the subject of ongoing studies. Both specific antibodies and B cells may contribute to control of primary infection or vaccination-induced protection in some circumstances, particularly against lower virulence Francisella strains. Thus a number of known proinflammatory and Th-1 T cell related components of the immune system combat this virulent bacterium; no doubt others remain to be discovered.

Innate and adaptive immunity to Francisella.

The hematopoietic system is composed of a remarkable variety of cells. Included are those circulating in the blood and their ancestors in marrow and progeny in the tissues. Also included are cells whose function is to remove both senescent cells from the bloodstream and any foreign material, especially microorganisms that may gain entrance to the body. After birth, the major location for hematopoiesis is the bone marrow, and in the newborn animal all medullary cavities of the skeleton are given over to this purpose. As the demands of body growth subside, hematopoiesis normally retreats to the metaphyses of long bones and to the flat bones of the pelvis, ribs, calvarium and vertebrae. From here, it may re-expand if need be, both into the bony cavities and even into extraskeletal sites such as the liver, spleen and lymph nodes. Hematopoietic stem cells are extravascular colonists in bone marrow and here they proliferate, differentiate and mature, being finally released as appropriately developed progeny into the circulation. These end cells have acquired sufficient membrane plasticity and movement to penetrate sinusoidal endothelium and leave the marrow. The hematopoietic cell system consists of a hierarchy in which the progenitor stem cells are capable of unlimited self-renewal and multilineal differentiation, giving rise to all blood cell types via committed precursor cells (Fig. 3.1). Committed precursor cells have a limited capacity for self-renewal and differentiation. The control of stem cell differentiation is little understood but is apparently initiated by the interaction of helper and suppressor lymphocytes and modulated by hormones specific for each cell line (Table 3.1).

Clinicopathologic Principles for Veterinary Medicine: The hematopoietic system

Mice vaccinated with the O-antigen of Francisella tularensis LVS lipopolysaccharide conjugated to bovine serum albumin develop varying degrees of protective immunity against systemic or aerosol challenge with virulent type A and type B strains of the pathogen.

Aerosol-, but not intradermal-immunization with the live vaccine strain of Francisella tularensis protects mice against subsequent aerosol challenge with a highly virulent type A strain of the pathogen by an αβ T cell- and interferon gamma- dependent mechanism

Live attenuated Francisella tularensis vaccine induced long-lasting humoral and cell-mediated immune responses in all 13 subjects studied. Lymphocyte blast transformation reactivity to F. tularensis appeared 2 weeks after vaccination in most subjects and remained unchanged for up to 1.5 years. Similarly, in most recipients, antibodies against F. tularensis were detectable by both the enzyme-linked immunosorbent assay (ELISA) and the agglutination method from 2 weeks after vaccination, although diagnostically significant agglutination titers (greater than or equal to 80) were not detectable until 4 weeks after vaccination. Maximal agglutination titers of 80 to 2,560 appeared at 4 to 8 weeks, and in spite of decreasing tendency, titers as high as 320 were still present 1.5 years after vaccination. ELISA showed the simultaneous, but not parallel, appearance of different immunoglobulin classes, immunoglobulin M (IgM) reaching individual maximal values 1.8 months after vaccination on average, at the same time as agglutinating antibodies, 1 week earlier than IgA, and about 1 month earlier than IgG. All of these immunoglobulin classes persisted in significant amounts up to 1.5 years, with IgG generally dominant. Long-lasting IgA and IgM responses after vaccination, as also after infection, suggested that the serodiagnosis of tularemia generally requires two consecutive serum samples with a significant increase in the titer.

P Koskela

Papers

Cell-mediated and humoral immunity induced by a live Francisella tularensis vaccine.