Temporal fate mapping reveals age-linked heterogeneity in naive T lymphocytes in mice

Abstract

Understanding how our T-cell compartments are maintained requires knowledge of their population dynamics, which are typically quantified over days to weeks using the administration of labels incorporated into the DNA of dividing cells. These studies present snapshots of homeostatic dynamics and have suggested that lymphocyte populations are heterogeneous with respect to rates of division and/or death, although resolving the details of such heterogeneity is problematic. Here we present a method of studying the population dynamics of T cells in mice over timescales of months to years that reveals heterogeneity in rates of division and death with respect to the age of the host at the time of thymic export. We use the transplant conditioning drug busulfan to ablate hematopoetic stem cells in young mice but leave the peripheral lymphocyte compartments intact. Following their reconstitution with congenically labeled (donor) bone marrow, we followed the dilution of peripheral host T cells by donor-derived lymphocytes for a year after treatment. Describing these kinetics with mathematical models, we estimate rates of thymic production, division and death of naive CD4 and CD8 T cells. Population-averaged estimates of mean lifetimes are consistent with earlier studies, but we find the strongest support for a model in which both naive T-cell pools contain kinetically distinct subpopulations of older host-derived cells with self-renewing capacity that are resistant to displacement by naive donor lymphocytes. We speculate that these incumbent cells are conditioned or selected for increased fitness through homeostatic expansion into the lymphopenic neonatal environment.T-cell homeostasis | mathematical modeling | Ki67 | kinetic heterogeneity N ormal adaptive immunity depends on maintaining populations of naive CD4 and CD8 T cells of sufficient sizes and diversities of antigen receptors. Mature naive cells are generated by the thymus and, once in the periphery, divide slowly and are lost either to death or differentiation into effector cells. It is known qualitatively how both cytokine (1-7) and T-cell receptor (TCR) (4, 8, 9) signals influence their survival and self-renewal through division, but we still lack a quantitative understanding of the rules that govern the development and persistence of our naive T-cell repertoires.To develop our understanding of lymphocyte homeostasis, much effort has been directed at defining the kinetics of T cells under normal physiological conditions. Division and death are normally quantified by following the accumulation and loss of cells labeled in vivo with BrdU or deuterium from heavy water or deuterated glucose, taken up by dividing cells during administration of label and diluted following its withdrawal (10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20). These experiments are typically performed over days to weeks and collectively have revealed that cell populations initially assumed to be homogenous may in fact comprise multiple subpopulations dividing and dying at different rates (kinetic ...

Current best estimates for the average lifespans of mouse and human leukocytes: reviewing two decades of deuterium‐labeling experiments

“…Nevertheless, these analyses suggest that CD4 + RTE have a life expectancy of just 3 weeks (95% CI: 12‐36 days), and that only an estimated 27% of them mature into naive T cells with a life expectancy of 2‐3 months. The resulting overall life expectancy of CD4 + naive T cells of 1/(0.5 × 0.046 + 0.5 × 0.015) = 33 days is shorter than that of CD8 + naive T cells, which is in good agreement with other mouse studies as well as human studies …”

“…Hogan et al suggested that the MN CD4 + and CD8 + T‐cell pools in mice have another layer of kinetic heterogeneity, in that most cells are readily replaced by new RTE, while a subpopulation of long‐lived “incumbent” cells are resistant to displacement by RTE and maintain themselves by peripheral division (with interdivision times of 167 and 213 days for CD4 + and CD8 + naive T cells, respectively, estimated with a model for Ki67 expression). Our data were also suggestive for the existence of such long‐lived cells within the CD8 + MN T‐cell pool, as a relatively high enrichment of CD8 + naive T cells was retained during the de‐labeling phase of the prenatal labeling experiments.…”

Memory CD4 T cell subsets are kinetically heterogeneous and replenished from naive T cells at high levels

“…Specifically, we modelled the kinetics of replacement of host cells by donor cells within the CD4 + CD25 − CD44 hi CD62L hi  and CD4 + CD25 − CD44 hi CD62L lo populations, termed CD4 central memory (CD4 T CM ) and CD4 effector memory (CD4 T EM ) respectively (Figure 2A), for over a year post-treatment. We previously demonstrated that the kinetics of lymphocyte replacement in the busulfan chimeras are a rich source of information regarding homeostatic turnover and population substructure (Hogan et al, 2015).

10.7554/eLife.23013.006Figure 2.Estimating constitutive rates of generation of CD4 T cell memory.( A ) Gating strategy for CD4 central and effector memory subsets.

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“…These quantities were logit-transformed to normalise residuals. Using the maximum likelihood estimates of the variance of the residuals in each of the timecourses, fitting a model of temporal heterogeneity to the BrdU/Ki67 timecourses therefore required maximising the product of the likelihoods of the two timecourses (See Supporting Information S2 in Hogan et al (2015) for a description) over four continuous parameters ($\beta$, $ϵ$, ${\delta }^{+}$, $\tau$) and two discrete ones ($\hat{k}$, $\hat{b}$).To model multiple subpopulations (kinetic heterogeneity, KH) we used replicates of Equations 14. This process increased the number of parameters to be estimated and so we explored a model of two distinct subpopulations only.…”

Establishment and Maintenance of the Human Naïve CD4+ T-Cell Compartment

“…At thymus egress, naïve CD4 + T-cells feature unique phenotypic and functional properties (2, 13, 16, 76, 120). Moreover, in agreement with studies using manipulated murine models, the cell-intrinsic properties in terms of turnover, survival, and threshold for TCR activation is also modulated by prolonged time span in circulation (2, 8, 10–12, 121).…”