Chagas disease, caused by the flagellate parasite Trypanosoma cruzi affects 8–10 million people in Latin America. The mechanisms that underlie the development of complications of chronic Chagas disease, characterized primarily by pathology of the heart and digestive system, are not currently understood. To identify possible host genetic factors that may influence the clinical course of Chagas disease, Human Leucocyte Antigen (HLA) regional gene polymorphism was analyzed in patients presenting with differing clinical symptoms.
Two hundred and twenty nine chronic Chagas disease patients in Santa Cruz, Bolivia, were examined by serological tests, electrocardiogram (ECG), and Barium enema colon X-ray. 31.4% of the examinees showed ECG alterations, 15.7% megacolon and 58.1% showed neither of them. A further 62 seropositive megacolon patients who had undergone colonectomy due to acute abdomen were recruited. We analyzed their HLA genetic polymorphisms (HLA-A, HLA-B, MICA, MICB, DRB1 and TNF-alpha promoter region) mainly through Sequence based and LABType SSO typing test using LUMINEX Technology.
The frequencies of HLA-DRB1*01 and HLA-B*14:02 were significantly lower in patients suffering from megacolon as well as in those with ECG alteration and/or megacolon compared with a group of patients with indeterminate symptoms. The DRB1*0102, B*1402 and MICA*011 alleles were in strong Linkage Disequilibrium (LD), and the HLA-DRB1*01-B*14-MICA*011haplotype was associated with resistance against chronic Chagas disease.
This is the first report of HLA haplotype association with resistance to chronic Chagas disease.
Chronic Chagas disease consists of four different forms categorized on the basis of their clinical manifestations, namely; cardiac, digestive, cardiodigestive and indeterminate. In Latin America, there are 8–10 million seropositive persons who are at risk of, or have already developed serious clinical complications and who have limited access to effective treatment. The cardiac and digestive forms are characterized by tissue damage caused by persistent infection of Trypanosoma cruzi and are thought to be modulated by host immunity. In our large scale screening for chronic Chagas disease in Santa Cruz, Bolivia, hearts and colons of 229 seropositive patients were examined. We found 31.4% of patients had abnormal electrocardiograms (ECGs), 15.7% presented with megacolon, 5.2% had a combination of abnormal ECG and megacolon, and 58.1% were of indeterminate status. Previously, we attempted to ascertain whether parasite genetic polymorphism might account for the differences in clinical manefestations, by analyzing parasite DNA taken from the same study group (with the addition of a further 62 megacolon post-operational patients). We found no relationships between parasite lineages and clinical disease form. The present study reveals that host HLA polymorphisms associate with clinical manifestations of Chagas.
Citation: del Puerto F, Nishizawa JE, Kikuchi M, Roca Y, Avilas C, Gianella A, et al. (2012) Protective Human Leucocyte Antigen Haplotype, HLA-DRB1*01-B*14, against Chronic Chagas Disease in Bolivia. PLoS Negl Trop Dis 6(3): e1587. doi:10.1371/journal.pntd.0001587
Editor: Helton da Costa Santiago, National Institutes of Health, United States of America
Received: September 2, 2011; Accepted: February 11, 2012; Published: March 20, 2012
Copyright: © 2012 del Puerto et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: This study was supported in part by the Grant-in-Aid for 21c COE program (2003–2008), the Global Center of Excellence (GCOE) Program (2008–2011), Nagasaki University and Grant-in-Aid for Research A (23256003) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors have declared that no competing interests exist.
Following an extensive control program consisting of vector control, serological screening in blood banks and identification and treatment of congenital transmission, the estimated number of people infected with Trypanosoma cruzi, the causal agent of Chagas disease in Latin America, has fallen from approximately 20 million in 1981 to around 8–10 million in 2009 , . Most of the seropositive patients are chronically infected and more than 10,000 deaths are estimated to occur annually from the disease , , .
Cardiac myositis and autonomous neuroplexus degeneration of the digestive tract are major histopathological alterations that can arise during Chagas disease, and may lead to cardiac failure, digestive abnormalities, megacolon or megaesophagus. Based on these pathologies, there are often considered to be three major clinical forms of Chagas disease; cardiac, digestive and indeterminate , . This variation in pathological manifestation has been reported to relate to differences in host immune response, such as the ability to control parasitaemia, the strength of inflammatory reactions, and the induction of autoimmune like responses –. Indeterminate phase T-cells have been reported to correspond with modulatory responses such as increased IL-10 production by CD4+CD28− T cells and the expression of CTLA-4, a ligand that is involved in modulation of T-cell responses by CD8+ T-cells , . Additionally, the unregulated production of IFN-gamma by CD8+ T-cells in cardiac Chagas disease patients has been reported, which might result in the destruction of heart tissue due to its cytotoxic effect .
The highly polymorphic HLA Class I and II molecules determine the efficiency of T. cruzi epitope presentation to T lymphocytes that could affect the clinical course of Chagas disease , . Several HLA alleles and haplotypes have been reported to be associated with Cardiac Chagas disease in Chile, Venezuela, Brazil and Guatemala –. The HLA region contains not only classical HLA genes but a wide variety of immunologically relevant genes, such as nonclassical class I genes (MICA, MICB) , , and class III genes (TNF-alpha, beta), that may be involved in pathogenesis , .
In the present study, we investigated HLA class I (A, B, MICA, MICB), Class II (DRB1) and Class III (TNF-alpha) gene polymorphism in seropositive chronic Chagas patients in Bolivia, characterized by electrocardiogram (ECG), barium enema colon X-ray examinations and/or surgical operation.
Materials and Methods
The study subjects were described previously . Two hundred and ninety one patients with chronic Chagas disease (136 men and 155 women, mean age 45 years) were recruited from the Centro Nacional de Enfermedades Tropicales (CENETROP) (91 men and 119 women), Hospital Primero de Mayo (12 men and 7 women) and from post-operative patients at the Hospital Universitario Japonés (HUJ) (33 men and 29 women) in Santa Cruz, Bolivia.
Upon medical examination of patients other than the HUJ patients, if serological tests (Indirect Haemaglutination assay (IHA) and Indirect Immunofluorescence test (IIF) ) were positive, they were asked to participate in the study and signed informed consent was obtained. Informed consent was also obtained for the HUJ post-operative patients, using the same protocol.
ECG abnormalities were diagnosed based on the Minnesota Code Criteria and were confirmed independently by two cardiologists. Colon X-ray with barium enema examination was performed for the detection of megacolon. To exclude the possibility of including individuals who were asymptomatic upon examination, but who may not have had adequate time post-infection for symptoms to become apparent, those under 30 years of age were excluded.
Finally, 229 seropositive Chagas outpatients in Santa Cruz, Bolivia, were examined by ECG and/or barium enema colon X-ray as shown in Table 1. The 62 post-operational patients from HUJ were confirmed to be suffering from Chagas megacolon during the admission period.
The experimental protocol was approved by the Institutional Ethical Review Committee of the Institute of Tropical Medicine, Nagasaki University (No. 0210170018) and the Centro Nacional de Enfermedades Tropicales (CENETROP).
Genomic DNA was extracted from 10 mL of whole blood containing 10 mM EDTA using a DNA extraction kit (QIAGEN GmbH, FRG) and was stored at −20°C until use.
Typing of MICA, MICB, TNF alpha promoter region and GCT repeat polymorphism in the MICA gene transmembrane region (MICA-TM)
MICA, MICB, TNF-alpha promoter region and MICA-TM typing was performed as previously described –. Sequences were obtained from a 3730 DNA Analyzer (Applied Biosystems, USA) and submitted to the Assign- software ATF (Conexio Genomics ATF, Australia) for allele identification. The TNFP alleles were determined according to Higuchi T et al (1998) and Ubalee R et al (2001) , .
MICA-TM alleles were typed by sequencing on a 3730 DNA analyzer (Applied Biosstems, USA) and analyzed with the GeneMapper Software Version 3.7 (Applied Biosystems, USA). The fluorescent primers, 5′ F (GCC CAG TGT ATA ACA AGT CCC-6-FAM) and 3′ R (CCT TAC CAT CTC CAG AAA CTG C) were used.
HLA-A, HLA-B, HLA-DRB1 typing
Typing was carried out according to the manufacturer's specification for LABType SSO Typing, testing for each locus using LUMINEX Technology (ONE LAMBDA, INC, USA) and the retrieved output was analyzed by HLA Fusion software (ONE LAMBDA, INC, USA) for allele identification.
Statistical analysis was performed at the two and four digits levels. Allele frequencies less than 5% were removed from the analysis. The statistical significance and odds ratio (OR) of allele frequencies between each group was determined by Chi square and Fisher's exact tests using the StatsDirec software (StatsDirect Ltd, UK). P-values were considered significant when <0.05 following Bonferoni correction (Pc). Hardy-Weinberg Equilibrium, linkage disequilibrium (LD) and Haplotype analyses were calculated with PyPopWin18.104.22.168 software .
Frequencies of HLA-B*14 and HLA-DRB1*01 significantly decreased in megacolon patients compared with indeterminate patients
In the two digits analysis (Table S1, S3, S5), we observed a significant decreased frequency of alleles DRB1*01 and B*14 in the megacolon patients compared with indeterminate patients (Table 2). The frequency of HLA-B*14 was also significantly lower in the patients with ECG alteration compared to the indeterminate patients. In the four digits analysis (Table S2, S4, S6), DRB1*01:01, *01:02 and B*14:02 had the same tendency towards lower allele frequencies in the megacolon patients compared with the indeterminate patients. The frequency of the HLA-B*14:02 allele was significantly lower in the ECG alteration and/or megacolon patients compared with those of indeterminate symptoms (Table 2). The frequency of MICA*011 was also significantly lower in the complication positive groups (Table 2, S7).
TNF-alpha promoter region polymorphism and MICB alleles
Linkage disequilibrium (LD) between the alleles of HLA-DRB1, A, B, and MICA, B loci
Linkage disequilibrium (LD) was calculated for all combinations of alleles (Table S1, Table S2, Table S3, Table S4, Table S5, Table S6, Table S7, Table S8, Table S9), and significantly strong LDs was observed between the alleles, HLA-DRB1*01, A*33:01, B*14:02 and MICA*011 as shown in Table 3. Except for HLA-A*33:01, all the linkage group alleles showed the same tendency of being associated with protection against megacolon (Table 2). HLA-A*01:01 showed strong LD with HLA-B*08:01, DRB1*0301 and MICB*008. Although HLA-A*01:01 showed a non-significant tendency of association with non-megacolon patients, none of the LD group alleles showed any similar tendency (Table 2). No LD was observed between any HLA alleles and TNF-alpha promoter haplotypes.
We have previously shown that there was no association between T. cruzi lineage or sub lineage and the clinical manifestation of Chagas disease in samples from Bolivia . Here, we have analysed the same samples in order to determine if there were any associations between clinical disease and host genetic variation. We analysed polymorphic genes located in the MHC region, consisting of three sub regions; class I, II and III. Human MHC, HLA class I and II molecules play a crucial role in determining individual acquired immune responsiveness through the presentation of pathogen-derived peptides to CD8+ and CD4+ T-cells. In the class III sub region, there are a variety of genes related to immunity such as complement, TNF-alpha, Lymphotoxin, etc. .
Our study revealed that the HLA-DRB1*01-B*14-MICA*011 haplotype was significantly associated with protection from Chagasic megacolon. As the linkage disequilibrium between the HLA-B*14 and DRB1*01 was strong (Table 3), it was difficult to determine the primary associated locus within the haplotype.
HLA-class I is the antigen-presenting molecule on the cell-surface of infected host cells which stimulates microbe-derived antigen specific CD8+ T-cells. Therefore, HLA-B*14 itself could be directly related to protective T-cell immunity as was suggested by the Chagas disease mouse model .
If HLA-B*14 is more efficient at stimulating protective T-cells through the binding of antigenic peptides, other HLA-class I molecules that share the same antigen binding motif should show the same protective association. HLA-B*14 consists of three 4-digit alleles, HLAB*14:01, 14:02 and 14:06 that share the same antigen binding motifs but the latter two alleles are so rare that it was impossible to analyse their effect. As the HLA-B*14 alleles belong to the B27 supertype group that share the same anchoring residues in the peptide binding groove, we analyzed the member alleles B*38:01, 39:04, 39:05. 39:06, 39:14, 48:01, 48:03 for their total effect on protection against complication, and found no association (Tables S6 and S10) . This finding did not, therefore, support the hypothesis that the association between HLA-B*14 and protection against clinical Chagas disease is driven by the ability of the gene product to more effectively stimulate protective T-cells than other alleles of this gene.
The peripheral human and mice CD8+T-cells reactive to Trypanosome antigens have been identified . Interestingly, HLA-A*02:01 restricted epitopes from cruzipain and FL-160 were frequently recognized by PBMC of patients with Cardiopathy . Moreover, an HLA-A2 tetramer experiment showed that the number of IFN-gamma producing amastigote-specific CD8+ T-cells inversely correlated with the severity of the disease . It will be interesting to see if the same phenomenon occurred in the HLA-B*14 patients.
Immuno-regulatory mechanisms have been reported to be associated with clinical forms including Treg cells , NKT cells and NK cells . Whereas the HLA involvement in the induction of Treg cells is not yet clear, HLA-class I can interact with NK cells to suppress their activity through various inhibitory receptors such as KIR. HLA-B*14 belongs to the Bw6 family, the members of which preferentially stimulate specific members of the KIR family , , which could regulate NK cell activity during inflammation. HLA-non classical class I, MICA*011, which was closely linked to HLA-B*14 and DRB1*01 might also be functional as MICA is known to stimulate gamma-delta T-cells in the gut mucosa; a phenomenon that could relate to megacolon .
HLA-class II can present antigen to CD4+ T-cells so HLA-DRB1*01 may also be directly involved in the pathogenesis as well as HLA-B*14. Many autoimmune diseases are reported to be associated with specific HLA-class II alleles . Auto-reactive processes that involve the activation of cytokine producing T-cells may occur during infection. As was previously suggested, autoimmune mechanisms in the pathogenesis of chronic Chagas heart and colon may be regulated by the HLA-class II.
We analyzed 4-digit HLA-DRB1 alleles for association with Chagas disease clinical manifestations. As shown in Table 2, the HLA-DRB1*01 group included three alleles, DRB1*01:01, 01:02, 01:03 and all of them showed the same protective tendency when compared between the megacolon and indeterminate symptom groups. As two of them, 01:01 and 01:02 shared the same peptide-binding motif , we considered that the HLA-DR molecule itself was functionally related to resistance to megacolon. It was previously reported that the DRB1*01 allele was associated with susceptibility to Chagas cardiomyopathy in Venezuela . This opposite association to the megacolon resistance observed in the present work requires further clarification.
Despite the strong LD shown within the HLA-DRB1*01-B*14 haplotype, there was no strong linkage between TNF-alpha promoter alleles that may influence the levels of its production by immune cells . However, between HLA-DRB1 and HLA-B loci, 1270 kb of class III sub region containing more than 60 genes such as complements, heat shock proteins, 21-hydroxylase, are present that might be relevant to pathogenesis. The HLA-B*14:02-DRB1*01:02 haplotype was reported to be associated with V281L polymorphism in 21-hydroxylase in African-American and Caucasian populations . The same kind of abnormality was also reported in the HLA-A*01:01-B*08-DRB1*03 haplotype that is associated with several diseases such as allergy and viral infectious diseases . It was not significantly associated with Chagas. However, HLA-A*01:01 also showed a decreased frequency in the megacolon patients. Whole sequencing of the class III region of the associated haplotypes would be the next target for clarification of any genetic resistance.
We have no information relating to the immunological characteristics of the individuals who poscessed those haplotypes that might be associated with lymphocyte activation during a chronic infection. We did, however, analyze the relationship between individuals' specific antibody titers and their HLA alleles (data not shown) but there was no clear correlation. About 7.1% of the seropositive indeterminate individuals were estimated to carry this haplotype; therefore further to identify its characteristic immunological function is feasible. To our knowledge, this is the first report of resistant HLA haplotype association with chronic Chagas diagnosed by the active examination of silent colon lesion.
The frequency of the Alleles of HLA-DRB1 locus. Two digits analysis.
The frequency of the Alleles of HLA-DRB1 locus. Four digits analysis.
The frequency of the Alleles of HLA-A locus. Two digits analysis.
The frequency of the Alleles of HLA-A locus. Four digits analysis.
The frequency of the Alleles of HLA-B locus. Two digits analysis.
The frequency of the Alleles of HLA-B locus. Four digits analysis.
The frequency of the Alleles of MICA locus.
The frequency of the Alleles of MICB locus.
The frequency of GCT triplet polymorphism in the MICA-transmembrane region.
The frequency of B Supertype.
The frequency of TNF-alpha promoter region polymorphism.
We are grateful to Dr. Richard Culleton, Dr. Tetsuo Yanagi at NEKKEN Nagasaki University, and Dr. Graciela Russomando at IICS University of Asuncion Paraguay, for their critical discussion and suggestion.
Conceived and designed the experiments: JEN FUGV SM KH. Performed the experiments: FdP JEN MK YR CA AG JL FUGV. Analyzed the data: MK YR NK KM KH. Contributed reagents/materials/analysis tools: JEN YR FUGV KH. Wrote the paper: FdP JEN MK FUGV KM KH. Diagnostic of Chagas: JEN YR CA AG JL FUGV SM NK KM. Genetic analysis: FdP MK KH.
- 1. Quantative estimation of Chagas disease in the Americas. Montevideo, Pan American Health Organization (2006) (OPS/HDM/CD/425-06).
- 2. Chagas disease control and prevention in Europe. Report of a WHO Informal Consultation (2009) Geneva, Switzerland, 17–18 December. Geneva: World Health Organization, 2010 (WHO/HTM/NTD/IDM/2010.1).
- 3. WHO (2002) WHO Control of Chagas disease: second report of a WHO expert committee. 905 p. WHO technical report series.
- 4. First WHO report on neglected tropical diseases: working to overcome the global impact of neglected tropical diseases (2010) (WHO/HTM/NTD/2010.1).
- 5. Marin-Neto JA, Rassi A Jr (2009) Actualizacion sobre la cardiopatia de la enfermedad de Chagas en el primer centenario de su descubrimiento. Rev Esp Cardiol 62(11): 1211–1216.
- 6. Rassi A Jr, Rassi A, Marin-Neto JA (2010) Chagas Disease. Lancet 375: 1388–1402.
- 7. Marin-Neto JA, Cunha-Neto E, Maciel BC, Simoes MV (2007) Pathogenesis of chronic Chagas heart disease. Circulation 115: 1109–1123.
- 8. Arce-Fonseca M, Ballinas-Verdugo MA, Reyes PA, Aranda-Fraustro A, Monteon VM (2005) Autoantibodies to human heart conduction system in Chagas' disease. Vector Borne Zoonotic Dis 5: 233–236.
- 9. Manoel-Caetano Fda S, Silva AE (2007) Implications of genetic variability of Trypanosoma cruzi for the pathogenesis of Chagas disease. Cad Saude Publica 23: 2263–2274.
- 10. Souza PE, Rocha MO, Rocha-Vieira E, Menezes CA, Chaves AC, et al. (2004) Monocytes from patients with indeterminate and cardiac forms of Chagas disease display distinct phenotypic and functional characteristics associated with morbidity. Infect Immun 72: 5283–5291.
- 11. Cunha-Neto E, Nogueira LG, Teixeira PC, Ramasawmy R, Drigo SA, et al. (2009) Immunological and non-immunological effects of cytokines and chemokines in the pathogenesis of chronic Chagas disease cardiomyopathy. Mem Inst Oswaldo Cruz Jul 104 (l)1: 252–258.
- 12. Menezes CA, Rocha MO, Souza PE, Chaves AC, Gollob KJ, et al. (2004) Phenotypic and functional characteristics of CD28C and CD28K cells from chagasic patients: distinct repertoire and cytokine expression. Clin Exp Immunol 137: 129–138.
- 13. Souza PE, Rocha MO, Menezes CA, Coelho JS, Chaves AC, et al. (2007) T cruzi infection induces differential modulation of costimulatory molecules and cytokines by monocytes and T cells from patients with indeterminate and cardiac Chagas disease. Infect Immun 75: 1886–1894.
- 14. Rao X, Costa AI, van Baarle D, Kesmir C (2009) A comparative study of HLA binding affinity and ligand diversity: implications for generating immunodominant CD8+ T cells responses. J Immunol 182(3): 1526–1532.
- 15. Alvarez MG, Postan M, Weatherly DB, Albareda MC, Sidney J, et al. (2008) HLA class I-T cell epitopes from trans-Sialidase proteins reveal functionally distinct subsets of CD8+ T cells in chronic chagas disease. PLoS Negl Trop Dis 2(9): e288.
- 16. Campbell DA, Westenberger SJ, Sturm NR (2004) The determinants of Chagas disease: connecting parasite and host genetics. Curr Mol Med 4(6): 549–562.
- 17. Fae KC, Drigo SA, Cunha-Neto E, Ianni B, Mady C, et al. (2000) HLA and ß-myosin heavy chain do not influence susceptibility to Chagas' disease cardiomyopathy. Microbe and Infection 2: 745–751.
- 18. Aida K, Juarez S, Kikuchi M, Gil M, Ayau O, et al. (2000) HLA-B35 and MICA-A5 synergistically enhanced susceptibility to Chagas Heart disease. MHC 7: 63–70.
- 19. Field SF, Nejentsev S, Walker NM, Howson JM, Godfrey LM, et al. (2008) Sequencing-based genotyping and association analysis of the MICA and MICB genes in type 1 diabetes. Diabetes 57(6): 1753–1756.
- 20. Mizuki N, Ota M, Kimura M, Ohno S, Ando H, et al. (1997) Triplet repeat polymorphism in the transmembrane region of the MICA gene: a strong association of six GCT repetitions with Behçet disease. Proc Natl Acad Sci U S A 94(4): 1298–1303.
- 21. Higuchi T, Seki N, Kamizono S, Yamada A, Kimura A, et al. (1998) Polymorphism of the 5′-flanking region of the human tumor necrosis factor (TNF-alpha) gene in Japanese. Tissue Antigens 51: 605–612.
- 22. Ubalee R, Suzuki F, Kikuchi M, Tasanor O, Wattanagoon Y, et al. (2001) Strong association of a tumor necrosis factor-alpha promoter allele with cerebral malaria in Myanmar. Tissue Antigens 58(6): 407–410.
- 23. del Puerto R, Nishizawa JE, Kikuchi M, Iihoshi N, Roca Y, et al. (2010) Lineage analysis of circulating Trypanosoma cruzi parasites and their association with clinical forms of Chagas disease in Bolivia. PLoS Negl Trop Dis 4(5): e687.
- 24. Lancaster A, Nelson MP, Single RM, Meyer D, Thomson G (2003) “PyPop: a software framework for population genomics: analyzing large-scale multi-locus genotype data”. In: Altman RB, et al., editor. Pacific Symposium on Biocomputing. vol. 8. : 514–525. Singapore.
- 25. Freitas JM, Andrade LO, Pires SF, Lima R, Chiari E, et al. (2009) The MHC gene region of murine hosts influences the differential tissue tropism of infecting Trypanosoma cruzi strains. PLoS One 4(4): e5113.
- 26. Sidney J, Peters B, Frahm N, Brander C, Sette A (2008) HLA class I supertypes: a revised and updated classification. BMC Immunology 9: 1.
- 27. Martin DL, Weatherly DB, Laucella SA, Cabinian MA, Crim MT, Sullivan S, et al. (2006) CD8+ T-Cell responses to Trypanosoma cruzi are highly focused on strain-variant trans-sialidase epitopes. PLoS Pathog 2(8): e77.
- 28. Fonseca SG, Moins-Teisserenc H, Clave E, Ianni B, Nunes VL, et al. (2005) Identification of multiple HLA-A*0201-restricted cruzipain and FL-160 CD8+ epitopes recognized by T cells from chronically Trypanosoma cruzi-infected patients. Microbes Infect 7(4): 688–697.
- 29. Laucella SA, Postan M, Martin D, Hubby Fralish B, Albareda MC, et al. (2004) Frequency of interferon- gamma -producing T cells specific for Trypanosoma cruzi inversely correlates with disease severity in chronic human Chagas disease. J Infect Dis 189(5): 909–918.
- 30. de Araujo FF, Vitelli-Avelar DM, Teixeira-Carvalho A, Renato Zuquim, Antas P, Assis Silva Gomes J, et al. (2011) Regulatory T Cells Phenotype in Different Clinical Forms of Chagas' Disease. PLoS Negl Trop Dis 5(5): e992.
- 31. Vitelli-Avelar DM, Sathler-Avelar R, Dias JC, Pascoal VP, Teixeira-Carvalho A, et al. (2005) Chagasic patients with indeterminate clinical form of the disease have high frequencies of circulating CD3+CD16-CD56+ natural killer T cells and CD4+CD25High regulatory T lymphocytes. Scand J Immunol 62(3): 297–308.
- 32. Lanier LL (2005) NK cell recognition. Annu Rev Immunol 23: 225–274.
- 33. Vyas Y, Selvakumar A, Steffens U, Dupont B (1998) Multiple transcripts of the killer cell immunoglobulin-like receptor family, KIR3DL1 (NKB1), are expressed by natural killer cells of a single individual. Tissue Antigens 52(6): 510–519.
- 34. Groh V, Steinle A, Bauer S, Spies T (1998) Recognition of stress-induced MHC molecules by intestinal epithelial gammadelta T cells. Science 279(5357): 1737–1740.
- 35. Thorsby E (2011) On the future of HLA. Tissue Antigens 78(4): 229–240.
- 36. Fu XT, Bono CP, Woulfe SL, Swearingen C, Summers NL, et al. (1995) Pocket 4 of the HLA-DR (alpha, beta 1*0401) molecule is a major determinant of T cells recognition of peptide. J Exp Med 81(3): 915–26.
- 37. Colorado IA, Acquatella H, Catalioti F, Fernandez MT, Layrisse Z (2000) HLA class II DRB1, DQB1, DPB1 polymorphism and cardiomyopathy due to Trypanosoma cruzi chronic infection. Hum Immunol 61(3): 320–325.
- 38. Dorak MT, Yee LJ, Tang J, Shao W, Lobashevsky ES, et al. (2005) HLA-B, -DRB1/3/4/5, and -DQB1 gene polymorphisms in human immunodeficiency virus-related Kaposi's sarcoma. J Med Virol 76(3): 302–310.
- 39. Price P, Witt C, Allcock R, Sayer D, Garlepp M, et al. (1999) The genetic basis for the association of the 8.1 ancestral haplotype (A1, B8, DR3) with multiple immunopathological diseases. Immunol Rev 167: 257–274.