The human peripheral leukocyte subset composition depends on genotype variation and pre-natal and post-natal environmental influence diversity. We quantified this composition in adults and neonates, and compared the median values and dispersal ranges of various subsets in them. We confirmed higher frequencies of monocytes and regulatory T cells (Tregs), similar frequencies of neutrophils, and lower frequencies of CD8 T cells, NKT cells, B1 B cells and gamma-delta T cells in neonatal umbilical cord blood. Unlike previous reports, we found higher frequencies of eosinophils and B cells, higher CD4:CD8 ratios, lower frequencies of T cells and iNKT cells, and similar frequencies of CD4 T cells and NK cells in neonates. We characterized monocyte subsets and dendritic cell (DC) subsets in far greater detail than previously reported, using recently described surface markers and gating strategies and observed that neonates had lower frequencies of patrolling monocytes and lower myeloid dendritic cell (mDC):plasmacytoid DC (pDC) ratios. Our data contribute to South Asian reference values for these parameters. We found that dispersal ranges differ between different leukocyte subsets, suggesting differential determination of variation. Further, some subsets were more dispersed in adults than in neonates suggesting influences of postnatal sources of variation, while some show the opposite pattern suggesting influences of developmental process variation. Together, these data and analyses provide interesting biological possibilities for future exploration.

Immune parameters show characteristic normal baseline levels and variance in the population. We characterised the degree of inter-individual and within-individual variation over one-year time period in 33 immune cell subsets by flow cytometry in peripheral blood mononuclear cells from 43 healthy young adult volunteers. Our analysis revealed that immune subsets that showed low variability between individuals also showed low short-term fluctuations within-individuals, as well as concordance in siblings. However, when baseline levels and degree of fluctuation were considered together, individuals failed to cluster into discreet groups. Together, the data reveal complex inter-relationships between immune subsets in individuals, and provide insights into the observed heterogeneity between individuals and between multiple immune subsets.

Memory T and B lymphocyte numbers are thought to be regulated by recent and cumulative microbial exposures. We report here that memory-phenotype lymphocyte frequencies in B, CD4 and CD8 T-cells in 3-monthly serial bleeds from healthy young adult humans were relatively stable over a 1-year period, while Plasmablast frequencies were not, suggesting that recent environmental exposures affected steady state levels of recently activated but not of memory lymphocyte subsets. Frequencies of memory B and CD4 T cells were not correlated, suggesting that variation in them was unlikely to be determined by cumulative antigenic exposures. Immunophenotyping of adult siblings showed high concordance in memory, but not of recently activated lymphocyte subsets. To explore the possibility of cell-intrinsic regulation of T cell memory, we screened effector memory-phenotype T cell (TEM) frequencies in common independent inbred mice strains. Using two pairs from these strains that differed predominantly in either CD4 TEM and/or CD8 TEM frequencies, we constructed bi-parental bone marrow chimeras in F1 recipient mice, and found that memory T cell frequencies in recipient mice were determined by donor genotypes. Together, these data suggest cell-autonomous determination of memory T niche size, and suggest mechanisms maintaining immune variability.

4344 Gene-environment interplay in immune phenotypes has been extensively studied using steady-45 state cellular immune profiles (1-3), functional immune responses (2,(4)(5)(6)(7)(8), post-vaccination 46 responses (9,10) and V,D and J usage biases in naïve and memory T and B cell compartments 47 (11). Some of these studies have identified genomic correlates associated with specific steady-48 state immune phenotypes (2) and vaccine responses (12). However, there are conflicting findings 49 regarding the relative importance of genetic versus environmental factors in regulation of 50 immune phenotype (1,2,13,14), warranting further investigations at the population-level in 51 humans and mechanistic studies in mice on regulation of individual immune phenotypes. 52 53 Memory subsets in T and B lymphocytes are immune populations that are generated in response 54 to past immunogen exposure. Immunological memory, providing long-term persistence of 55 antigen-experienced cells contributing to rapid and robust responses following re-exposure (15), 56 is likely to have evolved in an ecosystem where environmental challenges including repeated 57 infections would be the norm (16) and the persistence of long-lasting antigen-specific cells 58 generated during immune responses would confer survival advantage (17). However, it is 59 possible that larger memory lymphocyte pool sizes may carry costs such as restriction of space 60 for the more repertoire-diverse naive T cell compartment (18), attrition of pre-existing memory 61 (19), and other bioenergetic costs (20). Such selection may well result in 'optimum' sizes of 62 memory lymphocyte pools (21), and these pool sizes could show population diversity, depending 63 on the diversity of pathogens in the ecosystem and their exposure rates (22). Diversity in pool 64 sizes of memory lymphocytes in a population could thus be determined by a combination of 65 genetic variability and diversity of environmental exposures. 66 4 67 A number of mechanisms can be envisaged regulating the pool size of the memory T cell 68 compartment, including cumulative life-time antigen-exposure and re-exposure, antigenic 69 persistence, degree of expansion, cell survival, attrition and niche-space availability (21,23-26). 70 Immune cells occupy a limited niche space in lymphoid organs (27) or in the periphery (28). This 71 niche size could be a function of size and/or structure of supporting lymphoid tissue architecture 72 (29), along with intrinsic properties of cells occupying the niche. Similar determinants could 73 affect steady state levels of transient cell populations such as immediate-effector T cells (30) and 74 plasmablasts, but their steady state levels would be expected to fluctuate more with short-term 75 environmental changes. 7677 On this background, we have explored the possibility of gene-environment interplay affecting the 78 steady state pool sizes of lymphocytes post-activation. We have used peripheral blood leucocyte 79 immunophenotyping in serial bleeds from healthy young adult hum...