Genes and Immunity (2000) 1, 423–427 2000 Macmillan Publishers Ltd All rights reserved 1466-4879/00 $15.00 www.nature.com/gene Genetic influence on peripheral blood T lymphocyte levels MA Hall1, KR Ahmadi2, P Norman3, H Snieder2, AJ MacGregor2, RW Vaughan3, TD Spector2 and JS Lanchbury1 1Molecular Immunogenetics Unit, Department of Rheumatology, Division of Medicine, 5th Floor Thomas Guy House, Guy’s, King’s and St Thomas’ Hospitals School of Medicine, King’s College, Guy’s Hospital Campus, London SE1 9RT, UK; 2Twin Research and Genetic Epidemiology Unit, St Thomas’ Hospital, London SE1, UK; 3South Thames Tissue Typing, 3rd Floor, New Guy’s House, Guy’s Hospital, London SE1 9RT, UK T lymphocytes are a major component of the adaptive immune system. CD4 positive T cell subpopulations regulate B cell and macrophage effector function while CD8 positive T cells are largely responsible for anti-viral cytotoxic activity. The degree of natural variation in the levels and ratios of the various T cell subpopulations is a possible risk factor for the development of autommune disease, infectious disease and cancer. There is some evidence from studies of inbred strains of mice and humans which suggests that variation in T cell subpopulations is genetically influenced. However, family studies alone cannot distinguish between common environmental and shared genetic influences and provide less robust estimates of the heritability than twin studies. To comprehensively examine genetic influences on a selection of important T cell phenotypes, we investigated variation in levels of total lymphocytes, CD3+, CD4+, CD8+, CD3+CD4+, CD3+CD8+ lymphocytes and in CD4:CD8 ratio as a proportion of lymphocytes and of T cells using the classical twin model approach. Healthy female twin pairs were sampled from the St. Thomas’ UK Adult Twin Registry. A maximum of 103 monozygotic (MZ) and 186 dizygotic (DZ) twins aged 18–80 years participated in the study. Whole blood samples were analysed for T cell subsets by flow cytometry. The relative genetic contribution to these phenotypes was estimated using a variance components model-fitting approach. Heritability estimates were calculated of 65% for CD4:CD8 T cell and lymphocyte ratios, around 50% for absolute lymphocyte, CD3+ and CD4+ counts, and 56% for CD8+ numbers. Unique (rather than shared) familial environment explains the remainder of the variance. Genetic factors have a major influence on the variation in peripheral T cell subset numbers. Polymorphism dictating such variation should be taken into account when assessing risk factors for T cell immune-mediated disease with a genetic background. Genes and Immunity (2000) 1, 423–427. Keywords: T cells; genetics; immune system; heritibility Introduction ulatory activities. Studies of the impact of genetic vari- ation on the functioning of the immune system have up The human immune response is a key physiological sys- to now largely concentrated on qualitative issues of tem upon which mammalian survival depends and response and non-response. Experimental manipulation would be expected to be a crucial target for the action of of the murine genome using ‘knock out’ and transgenic natural selection. T cells are a fundamental component of technologies has enabled insight to be gained in systems the adaptive arm of this response and are involved in which are driven to phenotypic extremes. The effects of defence against pathogens including bacteria, viruses, CD4 and CD8 phenotypes are examples of the former2,3 1 fungi, protozoa, and multicellular parasites. CD4 posi- while useful insights into T cell development have been tive T cell subpopulations regulate B cell and macro- gained from rearranged T cell receptor transgenics as phage effector function while CD8 positive T cells are examples of the latter.4 responsible for anti-viral cytotoxic and some immunoreg- In contrast, the existence of genetically determined natural variation in levels of T cell subpopulations in mice and humans has received little attention. Studies in Correspondence: Dr MA Hall, Molecular Immunogenetics Unit, Depart- mice first showed that T cell subset representation was ment of Rheumatology, Division of Medicine, 5th Floor Thomas Guy 5 House, Guy’s, King’s and St Thomas’ Hospitals Schools of Medicine, strain dependent and therefore under genetic control. 6 King’s College, Guy’s Hospital Campus, London SE1 9RT, UK. E-mail: Amadori et al more recently studied the genetics of CD4 margaret.a.hallȰkcl.ac.uk and CD8 T cell ratios in healthy human nuclear families We are grateful to the Special Trustees of St Thomas’ Hospital and and concluded that CD4:CD8 T cell ratio was under gen- Gemini Genomics Limited for support in initiating this research. etic control. The degree of natural variation in the levels The Twin Research Unit also receives support from the Arthritis and ratios of the various T cell subpopulations may con- Research Campaign, the Wellcome Trust and the Chronic Disease Research Foundation. The experiments described in this paper com- tribute to part of the genetic risk for the development ply with the laws of the United Kingdom. of autommune disease, infectious disease and cancer. To Received and revised 17 June 2000; accepted 21 June 2000 explore this hypothesis we have measured a number of Genetic influence on peripheral blood T lymphocyte levels MA Hall et al 424 fundamental T cell parameters in humans in a large bution to the total variance of the T cell (1%, P Ͻ 0.001) cohort of monozygotic (MZ) and dizygotic (DZ) twins. and lymphocyte (2%, P Ͻ 0.001) CD4:CD8 ratios. From a genetic perspective, MZ twins share all their genes while DZ twins are similar to ordinary siblings in Discussion that they share on average 50% of their segregating genes. However, unlike sibs, use of DZ twins has the vir- In this study, we investigated the extent to which genetic tue of removing any confounding effects of age and con- and environmental influences control the absolute levels trols for differences in pre- and postnatal circumstances and cellular ratios of various T lymphocyte subpopula- of gestation and rearing. In this study we establish the tions using the classical twin model approach. The study degree of genetic and environmental control over a num- population was entirely female and the age range ber of peripheral T cell parameters using the classical covered the adult period from 18 to 80 years. Our data twin model approach. indicate that quantitative variation in absolute levels of peripheral blood T lymphocyte levels is under significant Results genetic control. Heritability, the proportion of total vari- ance due to additive genetic factors, ranged from 45%, The age range of the twins under study was 18–80 years. for the CD3+ T cells, to 65%, for the CD4:CD8 T cell ratio. The mean age of the MZ twins was 47.7 ± 15.6 years and The variance due to unique environmental differences, for the DZ twins it was 50.0 ± 13.0 years. Table 1 summar- ranged from 33%, for CD4:CD8 lymphocyte ratios, to 52% ises the number of complete twin pairs (N), the mean for lymphocyte and CD3+ T cell levels. Age was shown values (±s.d.), and the intra-class correlations (ICC) of the to have a small but significant effect on absolute levels various immune markers for MZ and DZ twins. of CD3+, CD8+ and CD3+ CD8+ T cells, as well as the The mean values and standard deviations for the MZ CD4:CD8 T cell and lymphocyte ratios and explained up and DZ twin populations were comparable, although to 4% of the observed variance. some means were slightly higher in the MZ group. On This is the first classical twin study of an adult popu- testing for the differences in the means between the MZ lation which investigates the relative influences of genes and DZ group we showed that although the differences and environment on the absolute levels of peripheral were significant for lymphocytes, CD3+, CD4+ and blood T cells. Previous studies have either examined CD3+CD4+, this did not significantly influence the herita- small nuclear families6 or twins at age 12.7 Amadori et al6 bility estimates. studied subjects across a wide age range while Evans et For both the lymphocyte and CD3+ T cell populations, al7 reported a twin study of broadly similar T cell pheno- the intraclass correlations suggest a genetic effect as the types and sample size. Our results agree with these stud- ICC for the MZ twins is greater than the DZ intraclass ies in showing that levels of T cell subpopulations and correlations. ratios are under significant genetic control. Interestingly, The results of the univariate model fitting are shown our heritability estimates are somewhat lower for equiv- in Table 2. The AE model (ascribing variance due to addi- alent T cell phenotypes than those of Evans et al7 obtained tive genetic and unique environment) represented the from a mixed sex group of 12-year-old twins. A possible best-fitting and most parsimonious model for lympho- explanation is that genetic control relaxes with increasing cyte, CD3+, CD8+, CD3+CD4+ and CD3+CD8+ populations age and the influence of unique environment becomes (C and D could be dropped without any significant more important with age through the cumulative change in ␹2). Age was shown to contribute significantly exposure to a range of environmnetal and self antigens. to the total variance of CD3+ (2%, P Ͻ 0.00001), In agreement with the family study, age had a small CD3+CD8+ (3%) cells and CD8+ (4%, P Ͻ 0.00001). The but significant influence on not only the T cell (age vari- absolute numbers of CD3+ and CD3+CD8+ lymphocytes ance ෂ1%) and T lymphocyte (ෂ2%) ratios but also on all decreased with age.