Genetic control of resistance to mycobacterial infection

March of Mycobacterium: miRNAs intercept host cell CD40 signalling

The disease tuberculosis is fatal if untreated. It is caused by the acid-fast bacilli Mycobacterium tuberculosis. Mycobacterium resides and replicates within the alveolar macrophages, causing inflammation and granuloma, wherein macrophage-T cell interactions enhance the inflammation-causing pulmonary caseous lesions. The first interactions between Mycobacterium and the receptors on macrophages decide the fate of Mycobacterium because of phagolysosomal impairments and the expression of several miRNAs, which may regulate CD40 expression on macrophages. While the altered phagolysosomal functions impede antigen presentation to the T cell-expressed antigen receptor, the interactions between the macrophage-expressed CD40 and the T cell-expressed CD40-ligand (CD40L or CD154) provide signals to T cells and Mycobacterium-infected macrophages. These two functions significantly influence the resolution or persistence of Mycobacterium infection. CD40 controls T-cell polarisation and host-protective immunity by eliciting interleukin-12p40, nitric oxide, reactive oxygen species and IFN-γ production. Indeed, CD40-deficient mice succumb to low-dose aerosol infection with Mycobacterium because of deficient interleukin (IL)-12 production leading to impaired IFN-γ-secreting T-cell response. In contrast, despite generating fewer granulomas, the CD40L-deficient mice developed anti-mycobacterial T-cell responses to the levels observed in the wild-type mice. These host-protective responses are significantly subdued by the Mycobacterium-infected macrophage produced TGF-β and IL-10, which promote pro-mycobacterial T-cell responses. The CD40-CD40L-induced counteractive immune responses against Mycobacterium thus present a conundrum that we explain here with a reconciliatory hypothesis. Experimental validation of the hypothesis will provide a rationale for designing anti-tubercular immunotherapy.

Patients exposed to Mycobacterium tuberculosis infection with a prominent IgE response

BACKGROUND AND AIMS

Even though it has been reported that chronic immune activation associated with intestinal helminthic infections results in a predominant IgE response, specific IgE antibodies that are also interleukin 4 (IL-4) dependent have been reported in tuberculosis patients; however, this fact has not been widely reported. This study was aimed at investigating the levels of circulating IgE in Warao (an indigenous population) of the Orinoco river delta, an area isolated from contact with the tubercle bacillus for millennia until the mid-1960s as compared to Creole (nonindigenous population).

METHODS

A total of 294 individuals were studied, 161 Warao and 136 Creole. Patient group was comprised of 86 Warao patients (WP) and 60 Creole patients (CP). Control group was comprised of 75 Warao controls (WC) and 76 Creole controls (CC). Total serum IgE and IgE and IgG(4) reactivities to M. tuberculosis antigens were measured by an enzyme-linked immunosorbent assay (ELISA).

RESULTS

Levels of total serum IgE were significantly elevated in WP (13002.0 ± 11200.0 IU/mL) and WC (2763.5 ± 2596.2 IU/mL) than in CP (385.9 ± 155.1 IU/mL) and CC (356.6 ± 157.5 IU/mL) (p <0.0001). Anti-PPD and anti-H37Rv IgE were significantly higher in WP (0.240 ± 0.145 and 0.230 ± 0.155) than in CP (0.127 ± 0.152 and 0.97 ± 0.103, respectively) and also between WC (0.240 ± 0.273 and 0.147 ± 0.158) and CC (0.115 ± 0.136 and 0.43 ± 0.46, respectively) (p <0.0001). Anti-PPD and anti-H37Rv IgG(4) did not show differences among groups; however, anti-H37Rv IgG(4) was affected by anti-TB treatment, which could be predictive of treatment outcome.

CONCLUSIONS

The findings suggest that for the Warao population there is an intrinsic propensity to produce a high IgE response, which could be incompatible with the protective response to M. tuberculosis.

Comparison of immune responses of mice immunized with five different Mycobacterium bovis BCG vaccine strains

Among the various parameters which may contribute to Mycobacterium bovis BCG vaccination efficiency, the choice of the vaccine strain may play an important role. In the present study, we therefore compared the immunogenicity of five different BCG strains that are commonly used for BCG vaccine production (Glaxo 1077, Japanese 172, Pasteur 1173P2, Prague, and Russian strains). The comparison of the growth capacity of these BCG strains in BALB/c and C3H mice demonstrated that a great difference exists between the capacity of various BCG strains to multiply and persist in target organs. A much lower recovery of BCG could be shown in mice immunized with Prague and Japanese BCG strains. T-cell responses of BCG-immunized mice were also examined by analyzing T-cell proliferative responses, cytokine production, delayed-type hypersensitivity responses, and cytotoxic activity. All these assays demonstrated that BCG immunization induced strong CD4+ T-cell responses, mostly of the Th1 type, as demonstrated by interleukin-2 and gamma interferon production. These studies also demonstrated that there are differences between BCG strains in stimulating these T-cell responses. A lack of induction of cytotoxic activity was observed following immunization with the Japanese strain. Lower anti-purified protein derivative antibody responses were also observed after intravenous or oral immunization with this BCG strain. Finally, the protective activity of these BCG strains was tested by measuring the capacity of immunized mice to eliminate recombinant Pasteur and Japanese BCG strains which expressed beta-galactosidase. The results of these experiments clearly demonstrated that the Prague and Japanese strains were unable to protect mice against a second mycobacterial challenge whereas mice immunized with the Glaxo, Pasteur, or Russian strain eliminated the recombinant BCG very efficiently. Altogether, the results of the present study strongly support the view that there are considerable differences in the immunogenicity of various BCG vaccine strains and that these differences may play a major role in BCG vaccination efficiency.

Influence of the Bcg locus on macrophage response to the dimorphic fungus Candida albicans

The Bcg/Ity/Lsh gene (candidate Nramp) controls natural resistance to several parasites, such as Mycobacterium bovis, Leishmania donovani, and Salmonella typhimurium. Using two macrophage (M phi) cell lines (B10R and B10S) derived from mouse strains congenic at Bcg, we found that M phi s from resistant mice (B10R M phi s) act more effectively against the two morphogenetic forms of the dimorphic fungus Candida albicans compared with M phi s from susceptible mice (B10S M phi s). Moreover, when assessed for tumor necrosis factor secretion in response to the hyphal form of C. albicans, B10R M phi s are significantly more effective at expressing this secretory function than are B10S M phi s, closely resembling the trend of response to lipopolysaccharide. Overall, these results provide insight into the influence of the Bcg locus on the M phi response to C. albicans.

The Bcg gene story

The genetic influences on the course of mycobacterial infections during epidemics and in endemic areas have always been suspected, but the precise nature of such genetic control and of the inherited mechanisms of susceptibility have been unknown. We have used methods of population genetics in the mouse to discover a single dominant autosomal gene (Bcg), which controls the susceptibility to various species of mycobacteria as well as to other intracellular parasites. The phenotypic expression of the Bcg gene has been defined as nonspecific macrophage activation for bactericidal function, resulting in the destruction of ingested intracellular parasites early following infection. Using recombinant inbred strains of mice, we have mapped this gene to the centromeric part of chromosome 1 and we have created a high resolution linkage map and, subsequently, a physical map in the close vicinity of this locus. A 400 kb bacteriophage and cosmid contig assembled within the genomic interval overlapping Bcg contained six novel transcription units. RNA expression studies showed that one of these genes (designated Nramp for "natural resistance associated macrophage protein"), was expressed exclusively in macrophages. Nramp encodes an integral membrane protein that has structural homology with known prokaryotic and eukaryotic transport systems, suggesting a macrophage-specific membrane transport function. Susceptibility to infection (Bcgs) in 27 Bcgs and Bcgr strains tested is associated with a Gly-105 to Asp-105 substitution within predicted transmembrane domain 2 of Nramp, making this gene a strong candidate for Bcg. The chromosomal segment in the vicinity of the Bcg gene has been conserved in the human genome (chromosome 2q). Linkage analysis between the phenotype of disease during a tuberculosis outbreak in an extended multisib Canadian Indian family and allelic variants of chromosome 2 has revealed a significant LOD score. This finding, together with the emerging information on almost total sequence homology between the murine and human Nramp genes suggests that this gene may be responsible for the phenotype of resistance or susceptibility to tuberculosis.

Cellular and humoral immune responses of cattle to purified Mycobacterium bovis antigens

Cellular responses to several purified antigens of Mycobacterium bovis were examined in experimentally infected cattle over a period of 36 months, using in vitro cellular proliferation and interferon-gamma assays. These antigens (12, 19, 22a, b, 24, 25, 30, 32, 39, 65 and 70 kDa) included the majority of M. bovis protein antigens described to date and are highly homologous to those purified from M. tuberculosis. Cellular responses were examined at 3-month time intervals during the 36-month course of infection. All purified antigens induced cellular immune responses in the infected animals. The onset and magnitude of response to individual antigens varied among the animals. At any specific time during the period of infection one or more antigens appeared to be immunodominant but the immunodominance profile changed as the infection progressed. Humoral immune responses were low or absent in the first half of the infection period, but increased substantially for some of the antigens during the second half. Variation was observed among the different animals as to which antigens they recognized.

Spleen cell cytokine secretion in Mycobacterium bovis BCG-infected mice

Three susceptible mouse strains, i.e., BALB/c (H-2d), C57BL/6 (H-2b), and major histocompatibility complex-congenic BALB.B10 (H-2b), were infected intravenously with 4 x 10(6) CFU of live Mycobacterium bovis BCG and analyzed 4 weeks later for in vitro spleen cell cytokine secretion in response to purified protein derivative (PPD), BCG culture filtrate (CF), BCG cellular extract, total BCG, the purified extracellular 30-32-kDa antigen (the fibronectin-binding antigen 85), or the intracellular 65-kDa heat shock protein. C57BL/6 and BALB.B10 mice produced 5- to 10-fold more gamma interferon and interleukin-2 (IL-2) when stimulated with CF, PPD, and antigen 85 than BALB/c mice did. When stimulated with BCG extract and whole BCG, gamma interferon and IL-2 levels were generally lower and comparable in the three strains. IL-4 was detected in spleen cell culture supernatants from infected BALB/c mice but not from C57BL/6 or BALB.B10 mice. IL-5 could not be detected. C57BL/6 and BALB.B10 spleen cells also produced more tumor necrosis factor alpha and IL-6 after stimulation with PPD and CF than BALB/c cells did. Finally, BCG vaccination generated efficient protective immunity in C57BL/6 and BALB.B10 mice but not in BALB/c mice. These data suggest that secreted mycobacterial CF antigens selectively induce a strong TH1 response in BCG-infected C57BL/6 and BALB.B10 mice, whereas in BALB/c mice this response is partly counterbalanced by TH2 cells.

MPB 64 possesses 'tuberculosis-complex'-specific B- and T-cell epitopes

We have developed monoclonal antibodies (MoAb) reactive with a protein from Mycobacterium tuberculosis of apparent molecular mass 24 kDa. This protein was shown to be identical with MPB 64 (Harboe et al.,) MoAb bound to four different epitopes of which two were restricted to the 'tuberculosis complex' and two were also found in mycobacteria not belonging to the 'tuberculosis complex'. The cross-reactive MoAb demonstrate that MPB 64 is present in more mycobacterial species than previously assumed. MPB 64 was shown to induce strong delayed type hypersensitivity (Dth) reactions in outbred guinea pigs immunized with M. tuberculosis and M. bovis bacille Calmette-Guérin (BCG). No reaction was observed in animals immunized with mycobacteria not belonging to the 'tuberculosis complex'. The Dth-inducing capacity of MPB64 was compared with that of another 24 kDa protein purified from M. tuberculosis and of the previously described 38 kDa protein. The Dth responses to these three antigens were further analysed in four inbred guinea pig strains. A genetic restriction of the ability of the animals to respond to MPB 64 as well as to the 38 kDa protein was observed.

Delayed-type hypersensitivity and cell-mediated immunity in the pathogenesis of tuberculosis

It is widely believed that cell-mediated immunity and the associated ability of macrophages to destroy or inhibit the bacillus are all that is required to control pulmonary tuberculosis. However, although cell-mediated immunity is a major host defense against the tubercle bacillus, it is fully effective only in one of the four stages of the disease. Here, Arthur Dannenberg describes the entire pathogenesis of tuberculosis, with illustrations from the rabbit model of M.B. Lurie. In addition, he documents that the delayed-type hypersensitivity reaction (producing tissue necrosis) greatly benefits the host by arresting the logarithmic growth of bacilli within immature macrophages.

Antibody repertoire against culture filtrate antigens in wild house mice infected with Mycobacterium bovis BCG

Wild house mice (Mus domesticus) captured in a Flemish pigsty were infected intravenously with 4 x 10(6) variable units of Mycobacterium bovis BCG and examined by Western blot analysis for IgG secretion against BCG culture filtrate (CF) antigens. Wild mice showed a marked individual variation in antibody pattern when tested 4, 6 and 8 weeks after infection. Some animals reacted to a wide range of antigens and others only to a limited number. Most wild mice recognized preferentially antigens with molecular weight of 24 kD, 32 kD, 37-38-40 kD, 65 kD and 82 kD, i.e. the major CF antigens known to be recognized by sera from BCG-infected inbred laboratory strains, BALB/c, DBA/2, CBA/Ca and C57BL/6. The 32-kD fibronectin-binding protein and the 65-kD heat-shock protein appeared as very immunodominant in wild mice. Furthermore, about 20-25% of the mice reacted strongly with a unique antigen of 35 kD estimated molecular weight, to which the tested inbred laboratory mice did not respond. Monitoring the size of the bacterial population in the spleen indicated that the BCG inoculum did not replicate in wild mice, suggesting that the Bcgr allele is expressed in this population.

Repertoires of antibodies to culture filtrate antigens in different mouse strains infected with Mycobacterium bovis BCG

Two susceptible (Bcgs) mouse strains, BALB/c and C57BL/6, were compared by Western blot (immunoblot) analysis for their immunoglobulin G response to 14-day-old BCG culture filtrate (CF) following intravenous infection with live Mycobacterium bovis BCG. The two strains demonstrated a completely different antibody repertoire. BALB/c antibodies were directed against a wide range of CF antigens between 20 and about 100 kilodaltons (kDa), with a preferential recognition of the 65-kDa heat shock protein and the 32-kDa fibronectin-binding protein. C57BL/6 sera, on the other hand, showed a much more restricted antibody pattern, almost exclusively directed against three antigens with estimated molecular sizes of 37, 38, and 40 kDa. Whereas the 37- and 38-kDa antigens were also recognized by BALB/c mice, the 40-kDa antigen was very intensely stained by C57BL/6 sera only. F1 mice had the restricted antibody pattern of C57BL/6 after one injection of BCG and had a hybrid BALB/c-C57BL/6 phenotype following a boost injection of BCG 2 months after the initial infection. Analysis of seven recombinant inbred strains derived from the BALB/c x C57BL/6 cross and of congenic mice differing in major histocompatibility complex-coding chromosome 17 fragments suggests that a gene in the K-IA region of the H-2 locus is associated with the preferential recognition of certain CF antigens. Inoculation with the same dose of killed BCG failed to elicit an antibody response to these filtrate antigens.

BCG: A modifier of immune responses to parasites

The use of BCG (Bacille Calmette-Guerin) as an adjuvant is well-established for vaccination against leprosy and tuberculosis. Dominique Frommel and Phillippe Lagrange discuss the effects of BCG in the control of parasite infections, particularly leishmaniasis, and the possibility of the development of anti-parasite recombinant BCG vaccines.