Chapter 6: Host Genetics and Susceptibility
by Howard E. Takiff
6.1. The difficulty in proving a genetic component for human susceptibility
6.1.1. Introduction
Tuberculosis (TB), “The White Plague” was a predominant public health problem in Europe and America in the 18th, 19th, and early 20th centuries, and considerable effort was spent trying to understand it. With the advent of effective antibiotic therapy in the ’50s, the prevalence of the disease, and research on it, declined precipitously. Since the late ’80s, however, there has been a resurgence of TB in urban settings in developed countries as well as in the developing world and Eastern Europe (Bloom 1992), and concomitantly, there has been a revival of research on TB and its causative agent, M. tuberculosis. Many of the questions investigated in the past are now being re-addressed at the molecular level.
One of the principal questions that occupied earlier researchers was the interplay of bacterial and host factors that determines who becomes infected and who develops TB. The discussion over the causes of TB goes back at least as far as the ancient Greeks and Romans, and basically consists of three different explanations: an inherited disorder; a contagious disease; and a disease caused by poor living conditions. Hippocrates thought it was inherited, while Aristotle and Galen believed it was contagious (Smith 2003). As the disease was most common in the urban poor, crowded into the rapidly growing cities of the recently industrialized Europe, social reformers of the time believed that TB was caused by the deplorable living conditions of the working class and rejected a contagious explanation. Although this chapter will present the published evidence supporting a heritable component to TB susceptibility, really all three explanations are correct and inter-related, which makes it difficult to separate, evaluate, and define the heritable genetic component.
While the discovery of the TB bacillus by Koch in 1882 disproved the notion that the disease had a purely hereditary etiology, or was caused solely by the unhealthy living conditions of the lower classes in the early industrial age (Hass 1996), several aspects of TB epidemiology are not explained by the germ theory and suggest that there are individual differences in susceptibility: not everyone exposed to M. tuberculosis becomes infected; even when infection can be demonstrated with a positive tuberculin skin test (TST), only about one in ten infected individuals becomes ill; the course of the disease varies in different individuals – before antibiotics some tuberculars died rapidly of “galloping consumption” while others recovered or lived a relatively long life with chronic disease; and some infected individuals develop the disease only many years after the initial infection (Rich 1951). Without treatment, TB is fatal in about half of the patients who develop the disease.
As the disease was more common in particular families and racial or ethnic groups, a heritable component to susceptibility was a plausible assumption, but one that has defied solid experimental proof, perhaps due to the difficulty in eliminating the confounding biases of environment and exposure. In 1912, the statistician Karl Pearson, attempting to demonstrate racial differences in TB susceptibility, stated the basic question, “We have to inquire whether persons living habitually in the same environment and with practically the same risk of infection have the same chance of developing phthisis whatever be their stock” [cited in (Puffer 1946)].
Since the mid ’80s, there have been many studies that have tried to identify genes that might be associated with TB susceptibility, as well as those testing the validity of published associations. While there are several recent reviews of the subject (Bellamy 2005, Bellamy 2006, Fernando 2006, Hill 2006, Ottenhoff 2005, Remus 2003), it is hard to come to definitive conclusions on most of the genes, because the accumulated literature is often contradictory. Studies showing that a polymorphism in a plausible gene is associated with TB susceptibility are often contradicted by subsequent work in other populations that finds no association. This has led to the recent publication of meta-analyses attempting to examine the body of published work on particular genes to determine whether a convincing consensus emerges (Kettaneh 2006, Lewis 2005, Li 2006). This chapter will attempt to summarize the current, inconclusive state of investigation on genetic determinants of TB susceptibility. In addition, it will review studies performed prior to the molecular era to illustrate the history of the field, which may help to clarify why finding genetic determinants has been elusive. It will focus on human susceptibility to M. tuberculosis, and will not consider susceptibility to leprosy (Geluk 2006, Schurr 2006) or other mycobacteria, except in the discussion of immune deficiencies (Casanova 2002).
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The basic epidemiological designs employed in studies of genetic association, in approximate decreasing order of confidence that the results obtained are free of the complicating influences of environment and exposure are: · twin studies comparing disease concordance in monozygotic vs. dizygotic pairs · family linkage studies that associate the occurrence of TB in family mem-bers with the inheritance of a particular genetic marker · case-control studies showing that, compared to controls, individuals with TB are different in some particular variable, such as exposure, race, HLA type, or the presence of polymorphisms in genes encoding elements of the immune system, such as cytokines or macrophage receptors, etc. · anecdotal reports of family or ethnic clusters of TB cases, suggesting an in-creased susceptibility As will be seen, the details and rigor of experimental design and the selection of control populations greatly affect the ability to discover associations, and the valid-ity of the results obtained. This tour of the literature on the genetic basis of human susceptibility to TB begins with a review of older family and twin studies that provide the basis for the belief that there is a significant component of genetic susceptibility to TB, and that show the difficulties in proving it. This is followed by an examination of racial differ-ences in TB susceptibility, and then a summary of immunological defects, both general and specific, that confer extreme susceptibility to mycobacteria. After this comes a review of studies associating specific genes with susceptibility to common TB: first those looking at different human leukocyte antigen system (HLA) alleles; then studies on other genes thought to be important in human defense mechanisms against TB. Finally, after a review of human studies of genes equivalent to those altering TB susceptibility in mice, and work employing genomic scans, is an at-tempt to summarize the state of the field and put it into perspective. While this tour is not exhaustive, it attempts to critically present most of the relevant published work. TB in famous families French Royal Bourbon Family Louis XIII-(1601-1643) died of galloping consumption TB affected: His wife His son – Louis XIV (1638-1715) Simon Bolivar (1783 – 1830) died of TB TB deaths: Father – Juan Vincente (1786) Mother – Maria de la Concepción (1792) Brontës Chronic TB Father (died 1861) TB deaths: His wife His four children: Charlotte (1816-1855) (“Jane Eyre”) Emily (1816-1848) (“Wuthering Heights”) Anne (1818-1848) (“Agnes Grey”) Patrick (1817-1848) Ralph Waldo Emerson (1803-1882) Chronic TB (Romantic and Transcendentalist Poet) TB deaths: Father Two brothers Chronic TB: One brother In 1949, a descendent wrote that TB had claimed lives and caused illness in 10 generations. Henry David Thoreau (1817-1862) died of TB (“Walden” “On Civil Disobedience”) TB deaths: Grandfather Father Sister 6.1.2. Early family and twin studies Many early studies of TB in families compared the cumulative incidence of disease in the offspring of couples where one, both, or neither had TB, also noting other family history of TB, and whether cases were sputum positive (Puffer 1946, Stocks 1928, Frost 1933). While these studies clearly demonstrated that living in a house with a tubercular person increased the chances of developing TB, most investiga-tors accepted that their results represented a combination of the effects of exposure and hereditary predisposition. Two examples illustrate the difficulty in separating these components. Stocks and Karn (Stocks 1928) devised a correlation coefficient based on sibling disease concurrence expected by chance. They then used family records of 4,000 Belfast TB patients to demonstrate an excess of sibling cases occurring in families with a prior history of TB, as evidence of an inheritable factor in susceptibility. Although the attempt was interesting in its design, it could not assure comparability of environment and exposure, as a tuberculous relative could have had a con-founding effect, either as a source of exposure or as a marker for lower socioeco-nomic status. Puffer (Puffer 1946) attempted to separate exposure from heredity by comparing the incidence of TB in the spouses of tuberculous individuals with that in their children and siblings. Although an increased incidence of TB in the spouse of spu-tum positive tuberculars suggested the importance of exposure, TB was more common in consorts who additionally had a family history of TB, suggesting the greater importance of familial susceptibility. To address the obvious criticism that the spouses could have been exposed in childhood from the affected relative, Puffer stated that two thirds had no known household contact, although the contact may have been forgotten or missed. Overall, due to the near impossibility of controlling for household exposure, the family studies failed to convincingly demonstrate a genetic predisposition. Twin studies (Table 6-1) have an experimental design that should control for the effects of environment and exposure more reliably, and several have studied in-heritance of TB susceptibility. Monozygotic twins are genetically identical, while dizygotic twins are only as genetically similar as other siblings. If it can be as-sumed that both types of twins will share the same environments and exposures, a difference in concordance rates of TB – both twins with TB or both healthy – be-tween the two types of twins can be attributed to the genetic components, even if multiple gene causality is suspected. The concordance in monozygotic twins can also serve as a measure of penetrance – the proportion of gene carriers who express the trait (Cantor 1992). In a large study (Kallmann 1943) performed in the United States (US) nearly three-fold greater concordance was found in monozygotic twins than in dizygotic twins, whether or not there was a history of exposure (69.2 % vs. 26.3 % with known exposure; 61.5 % vs. 12.7 % without known exposure). The concordance in dizygotic twins was the same as seen with other non-twin siblings. This study would appear to be solid evidence supporting hereditary influences, but it is weakened by several sources of potential bias specific to twin studies (Cantor 1992, Fine 1981) that are worth examining in detail because they again illustrate the difficulties in isolating genetic components from differences in exposure, and the importance of experimental design. First, to assure validity, all affected twin pairs in the base population must be ob-tained. Kallman and Reisner relied upon reporting of twins from the active patients in various TB treatment facilities in New York City and New York State, a proce-dure that could lead to reporting bias favoring “novel” concordant monozygotic pairs, especially if there is no assurance that all twin pairs were identified. The study states that 657 twin pairs were identified, but the final analysis contained only 308 cases of “reinfection” TB, without a clear explanation of the exclusion criteria. The validity of twin studies depends upon the assumption of the equiva-lence of environmental and exposure components, but in Kallman and Reisner’s study there were more monozygotic pairs with TB in their direct ancestry (36.6 % vs. 13.4 %). They also failed to report on whether twins were living together, which tends to be more common in monozygotic pairs, and would be a source of in-creased concordance for uniformity of exposure, or if TB was spread from one twin to the other. In addition, even though they mention that TB is more common in females in the age group 20-35 years, the percentage of females in the two groups was not reported where the twin pairs were clustered. Table 6-1: Twin studies Monozygotic Dizygotic Monozygotic Dizygotic Total Pairs Concordant pairs Reference N % N % N % N % Diehl 1936 80 39 125 61 52 65 31 25 Dehlinger 1938 12 26 34 74 7 58 2 6 Kallman 1943 78 25 230 75 52 66 53 23 Harvald 1956 37 26 106 74 14 38 20 19 Simonds 1963 55 27 150 73 18 32 21 14 The Prophit study set out to re-examine the conclusions of Kallman and Reisner’s study by trying to correct all its shortcomings (Simonds 1963). It exhaustively searched for all twins among active patients in English TB clinics, determined if the twin pairs were living together at the time of onset of TB in the index cases, whether the index case was sputum positive, and reported on the sex of all subjects. Although more concordance was found in monozygotic than in dizygotic pairs (32 % vs. 14 %), the authors believed that this difference could be explained by other factors: more female monozygotic than dizygotic twins (68 % vs. 43 %), especially in the susceptible 20-30 years age group; more monozygotic twins living together (58 % vs. 50 %); more TB concordance among those living together (42.4 % vs. 18.4 % for monozygotic, 16.3 % vs. 10.3 % for dizygotic); more spu-tum positive index cases among concordant pairs (72 % vs. 49 % for monozygotic, no difference for dizygotic); and more TB in parents of monozygotic than dizygotic twins (57.2 % vs. 43.5 %) – even though most of these differences were not statis-tically significant. Comstock’s re-analysis of the data (Comstock 1978), using multiple regression to control for the sex of co-twins, age at diagnosis, type of TB, sputum positivity of index twin, TB contact of co-twin, twins living together, and years between diagnosis of the twin pairs, still found a two-fold difference in con-cordance between twin types (31.4 % in evidence for monozygotic vs. 14.9 % for dizygotic; p < 0.05). Twin studies constitute the strongest evidence for a genetic component to TB sus-ceptibility because they control for bias better than any other experimental study design, and because there is relative consistency of the findings in most studies (Table 6-1) (Dehlinger 1938, Diehl 1936, Harvald 1956). A conservative conclu-sion might be that some inheritable component exists, but it has a maximal pene-trance of only 65 %, and the most careful study ever performed found only 31.4 % penetrance. In other words, in as few as only a third of cases, two individuals with exactly the same genes and similar exposures will either both develop or both not develop TB. 6.1.3. Racial differences Much of the controversy about genetic susceptibility to TB in the early part of the 20th century was concerned with allegations of racial differences, or more specifi-cally, that Asians and especially Africans and African Americans had less innate resistance than Whites. While the near fixation on this topic by authors such as Rich (Rich 1951) might be ascribed to the prevailing racism of the period, the as-sumption of greater susceptibility of Africans and African Americans continues to be cited in current literature, with investigators now using molecular findings to try to explain it (Liu 2006). While Rich gave equal credit to “the marked influence of environment… in different economic strata of individual communities within a given country” for Whites, he attributed the higher rates in Africans and African-Americans predominantly to the effects of genetic composition. Although he cited examples of higher TB rates in Africans, he really concentrated on the more severe nature of the pathology of the disease. He proposed that because of Africa’s short history of exposure to TB, Africans have not developed genetic resistance to the bacillus, and therefore many Africans, even as adults, develop a systemic, over-whelming form of the disease usually seen only in White children. While Rich states that he “has no intention of minimizing the importance of adverse economic and environmental conditions as factors that influence the TB mortality rate of the Negro,” one cannot help but recall the work of Dr. James McCune Smith in de-bunking the notion that African Americans were genetically predisposed to rickets by showing that whites of the same low socioeconomic status were similarly pre-disposed (Krieger 1992). Stead and Bates (Bates 1993, Stead 1992, Stead 1997) cite several examples to support their argument that Africans and Native Americans have less resistance to TB: the greater TB mortality of the Sudanese conscripted into the Egyptian army compared to the Egyptian soldiers; the similar fate of Senegalese soldiers sent to France in the first world war; and the decimation by TB of the American Indians forced to live on US military bases. It’s interesting that these three commonly cited examples all involve foreign conscripts or internees on a colonizer’s military base, and rely on the dubious assumption that their physical and emotional environments were the same as those of the host soldiers. Stead and Bates expound on the often-cited theory for the existence of racial differ-ences in susceptibility – the duration of the exposure to endemic TB in Africa and Asia has not been long enough to select for a resistant gene pool. They postulate that a TB epidemic has a 300-year cycle, in which the more resistant survivors reproduce and increase the proportion of naturally resistant individuals in the population, so that after 50-100 years, the mortality, and subsequently morbidity, reach a peak and then progressively decline. The White populations in Western Europe and the US, where the epidemic peaked in the late 1700’s and early 1800’s, are now composed of individuals with a relatively resistant genetic make-up. The Africans, Eskimos and other Native Americans, however, were only exposed to TB much later, so their gene pool has yet to complete the selection for resistant indi-viduals (Stead 1992). This theory, though still cited in current literature (Fernando 2006), is completely unproven and will likely remain so. Indeed, how could it be proven that the progressive lowering of rates for TB and other infectious diseases in Western Europe and the US, prior to the introduction of antibiotics, was the result of a changing gene pool and not of improvements in nutrition, housing, and working conditions, whose influences could outweigh any putative inheritable component (McKeown 1978)? Nonetheless, the abundance of literature describing increased susceptibility and a more progressive disease course in Africans and Native Americans suggests that some racial difference may, in fact, exist. Putting aside the theory for the origin of racial differences, are there any studies that have sufficiently controlled for environment and exposure, in order to credibly document a difference? Kushigemachi et al. critically reviewed the epidemiological studies that relate to this question (Kushigemachi 1984). They reasoned that the only studies that could provide usable information are those that follow TST-positive groups of people for the development of disease. They cite several relevant studies from the literature (see Table 6-2), mostly isoniazid (INH) prophylaxis or bacille Calmette-Guérin (BCG) vaccine trials. In the two studies done on Eskimos, the average annual case rates of 936 and 725 per 100,000 were much higher than rates seen in any other study, but there is no data on other risk factors. In studies predominantly involving Whites, the annual case rates varied from 29-79/100,000. The few BCG trials that included more than one race tended to show higher rates in Blacks than Whites, but both the absolute rates and the racial differences varied. In the Alabama study, the overall racial difference was predominantly due to very high rates in young Black women. The best single study was among Navy recruits, because the environment and follow-up were usually equivalent, at least once they were in the Navy. In that study, African Americans had an annual rate only 17 % higher than whites (91/78), but the Asians (195) had a rate more than double that of African Americans. It was also noted that upon entry into the Navy, highly positive purified protein derivative (PPD) reactions (> 20 mm) were more common in African Americans, which sug-gested that some may have entered with active disease. Because of the variability of the rates in Whites, the small difference found in the Navy study, and the lack of data on other risk factors, the authors concluded, “as-sertions that certain racial groups possess a “natural resistance” to TB are clearly unwarranted on the basis of available evidence.” The high rates in Asians and Eskimos compared to both Whites and Blacks seem less convincingly dismissed than the differences between Blacks and Whites, but risk factors, such as nutritional state, lack of a TB control program, or crowded and closed living conditions may explain the differences. In fact, after the implementation of intensive TB control measures in the Eskimo (Inuit) population in Canada, their TB rates, which had been the highest recorded in the world, showed the fastest rate of decline on record (Enarson 1986). Stead attempted to eliminate exposure and environmental bias by studying 1,786 documented TST conversions among 13,122 residents of integrated nursing homes in Arkansas, with a similar analysis of approximately 2,000 inmates from inte-grated prisons in Minnesota and Arkansas (Stead 1990). The results were analyzed using multivariate analysis with a proportional-hazards model, adjusting for covari-ates of age, sex, and percentage of nursing home residents who were TST positive at entry. They found that Blacks had a higher rate of TST conversions (7.2 % in Whites vs. 13.8 % in Blacks overall, P < 0.001) regardless of the percentage of Black residents of the facility, and regardless of the race of the potential source patient. In fact, Blacks had higher rates of TST conversions even when the pre-sumed source case was White (8.4 % vs. 15.3 %; P < 0.001). Table 6-2: Race differences and TB rates Location Criteria for a positive reactor Observa-tion period (years) Age on entry (years) Racial group Average annual case rates per 100,000 reac-tors Muscogee, George and Rus-sell Coun-ties, Ala-bama (20,21) > 5 mm indura-tion to 5 T.U. PPD (Mantoux) 20 20-29 WM 78 BM 74 WF 32 BF 96 30-39 WM 49 BM 75 WF 10 BF 93 40-49 WM 104 BM 97 WF 32 BF 83 50-59 WM 141 BM 131 WF 107 BF 105 Puerto Rico (22) > 6 mm indura-tion to 1 T.U. or 10 T.U. PPD- (Mantoux) 18,87 1-19 White 91 Black 87 U.S. Navy Recruits (23) > 10 mm indura-tion to 5 T.U. (Man-toux) 4 17-22 White 78 Black 91 Asian 195 WM: white males T.U. = tuberculin units BM: black males WF: white females BF: black females Similar results were found in the prison populations. In contrast, however, there was no racial difference in the incidence of TB that developed in the nursing home residents with positive skin tests. The authors interpreted this as evidence for the distinction of two aspects of TB, the initial infection and the development of dis-ease, and concluded that Blacks have decreased resistance to the initial infection, but that once infected, they develop TB at the same rates as TST-positive Whites. This is consistent with the conclusions of the review by Kushigemachi et al. Al-though the nursing home setting convincingly controls for sources of bias, includ-ing age and sex, there is no data on the residents’ weights, general health, or pat-terns of association and rooming. One other problem is that when no source patient was identified, the difference in TST conversion rates was greatest (4.4 % vs. 13.2 % P < 0.001), suggesting that the Blacks may have had some other source of infection, perhaps from visitors, which could explain all the differences. Even if African-Americans have a slightly increased rate of infection, the fact that there was no difference in the rate of progression to disease deflates the credibility of arguments that their immune system is less capable of controlling the infection. A separate study looked at TST conversion in school children exposed to a physical education teacher with TB. No racial differences were found, leading the authors to question the validity of the conclusions from the nursing home study (Hoge 1994). The notion that the decline in TB in Europe was due to genetic selection runs counter to most thinking in the public health field. In the ’70s, the historian Thomas McKeown (McKeown 1978) showed that the death rates in England and Wales from TB and other respiratory diseases declined precipitously from about 1830 to 1950, well before the advent of the BCG vaccine and anti-tuberculosis drugs (Fig-ure 6-1). A similar decline also occurred in the United States. McKeown concluded that improved nutrition was responsible for the decline in mortality and the increase in population, while others later argued that more im-portant factors were the general improvements in living standards and such public health measures as improved housing, isolation of infectious individuals, clean drinking water, and improved sanitation (Szreter 2002). Nonetheless, it is generally accepted that this dramatic decrease was mainly the result of societal factors. This explanation appears plausible because: the decline was temporally linked to the improvements in living conditions and public health; the decline was too rapid and steep to be explained exclusively by genetic selection (Lipsitch 2002); the rate of decline remained steep even as the putative “selective pressure” decreased; and the dramatic decline in mortality was not limited to TB, but was also seen for many other infectious diseases (McKeown 1978). Although an element of genetic selec-tion may also have played a role, the primacy of societal factors was demonstrated by the rise in TB rates in the US in the 1980s and ’90s that accompanied the in-crease in homelessness and decrease in TB control measures, and set the stage for the rampant spread of TB in the HIV-infected population (Frieden 1996). In New York City in the ’90s, it was found that infection with a clustered TB strain, con-sidered to be a marker of recent transmission, was associated with both homeless-ness and with being African-American. Can it then be argued that this demonstrates a genetic susceptibility to TB in the homeless? Taking into account the question-able hypothesis of extensive genetic selection for a TB-resistant population, and the lack of well-controlled, reproducible studies demonstrating that any racial group has either an increased susceptibility to infection or an increased propensity to develop disease, the notion of racial differences in susceptibility seems unproven. Nonetheless, it is certainly possible that distinct ethnic groups and populations may have different frequencies of polymorphisms that confer susceptibility or resistance to TB, and the frequency of alleles that conferred severe susceptibility in an en-demic setting may be reduced over time. However, the danger in this line of think-ing is that higher rates of TB in these populations may be accepted as the irremedi-able result of genetic make-up, rather than the consequence of lower socio-economic and health status along with poor TB control programs, which have the potential for improvement if, as demonstrated in New York City, sufficient finan-cial and political resources are committed to the task. Figure 6-1: Fall in deaths from TB in England and Wales from 1838 to 1970. Most of the drop occurred before specific treatment or immunization was available. (Scrimshaw 1976). Avail-able at: http://www.unu.edu/unupress/food/V181e/p07a.gif 6.2 Search for mutations and polymorphisms that increase susceptibility 6.2.1 Tuberculosis susceptibility in generalized immune deficiencies Considerable insight has been obtained by studying humans with immunological deficiencies, and determining which genetic defects lead to increased risk of myco-bacterial infections (Picard 2006). Undoubtedly, the largest group of highly sus-ceptible persons are individuals infected with the HIV virus, who are prone to de-velop TB early in the course of the disease. After the onset of AIDS, they are also susceptible to atypical or environmental mycobacteria as well as many other patho-genic and opportunistic agents. TB takes the lives of a large percentage of AIDS patients in Africa (Cantwell 1996), and the early susceptibility underlines the overwhelming importance of CD4+ T cells in immunity to TB. There are over 100 different primary genetic immunodeficiencies that predispose to infections with a variety of viruses, bacteria, fungi and protozoa, but only a few have been associated with severe mycobacterial infections (Casanova 2002). As might be imagined, children with severe combined immunodeficiency (SCID) who completely lack T cells are highly vulnerable to disseminated BCG infections after being vaccinated. Only a few cases of infections with atypical mycobacteria and M. tuberculosis have been described in these patients, but this may be due to lack of exposure, because without a bone marrow transplant, most of these children die within a year of birth. Disseminated BCG infection, pneumonia with M. intracellulare, and a M. tuber-culosis brain abscess (Metin 2004) have been described in individuals with hyper-IgE syndrome, a rare autosomal dominant disorder characterized by high serum IgE levels, eczema, and susceptibility to bacterial and fungal infections (Casanova 2002). Reports have described low levels of interleukin 12 (IL-12) and interferon gamma (IFN-g) in several of these patients (Netea 2005), but this defect must be mild or variable, as many hyper IgE patients have been vaccinated with BCG and survived into adulthood without mycobacterial infections. A patient was recently described, who had been clinically diagnosed with hyper IgE syndrome and was unusually susceptible to various microorganisms including mycobacteria, as well as virus and fungi (Minegishi 2006). A mutation was found in the gene for tyrosine kinase 2 (Tyk2), a non-receptor tyrosine kinase of the Janus kinase family. The patient’s cells showed defects in multiple cytokine signaling pathways, including IFN-g, which were restored by transducing an intact Tyk2 gene. The neutrophils of patients with chronic granulomatous disease lack the oxidative burst associated with ingestion of microorganisms, due to mutations in the NADPH oxidase complex. This defect in neutrophil killing makes them susceptible to severe recurrent bacterial and fungal infections. Both disseminated and local infections with BCG are fairly common in these patients (Jacob 1996), but disseminated in-fections with atypical mycobacteria (Moskaluk 1994, Ohga 1997), and TB (Barese 2004) have also been described, demonstrating that the phagocytic respiratory burst plays a role in the control of mycobacterial infections. Mutations that impair signaling and activation of gene transcription promoted by Nuclear Factor kappa B (NF-k-B) cause a rare disorder called anhydrotic ectoder-mal dysplasia with immunodeficiency. Affected patients are predisposed to dis-seminated infections with atypical mycobacteria, septicemia from pyogenic bacte-ria, and viral infections. This syndrome has been associated with X-linked hypo-morphic (reduced function) mutations in NF-k-B essential modulator (NEMO), and autosomal dominant hypermorphic mutations in the inhibitor of NF-k-B (von Bernuth 2005). Overall, mycobacterial infections occur in perhaps a third of patients with severe combined immunodeficiency and anhydrotic ectodermal dy