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By a balance of cell production and cell loss, B-cell and T-cell populations are generally maintained at a relatively stable size throughout life. Following lymphocyte-depleting therapies however, for example for the treatment of hematological malignancies, reconstitution of some lymphocyte subsets typically takes months to years. Such long-term immunodeficiency results in an increased susceptibility to infectious and malignant diseases and is associated with an increased mortality. Why recovery of these lymphocyte subsets takes so long is poorly understood. To be able to understand the process of immune reconstitution, and how for example advanced aging, and chronic infections affect the lymphocyte compartment, it is essential to first understand how lymphocyte populations are maintained in healthy conditions. The studies in this thesis were focused on investigation of B-cell and T-cell dynamics in healthy conditions and in conditions in which lymphocyte numbers are reduced. By performing in vivo stable isotope labeling studies in healthy volunteers and in lymphopenic stem-cell transplanted and kidney-transplanted patients, we studied how healthy aging, therapeutic lymphocyte depletion, and immunosuppressive therapy affect lymphocyte production and survival rates. In combination with T-cell receptor excision circle analyses and mathematical modeling, we also investigated the relative contributions of thymic output and peripheral cell division to maintenance of the naive T-cell population. The main findings of these studies are that (i) thymic output contributes very little to T-cell production from young adulthood onward, and declines approximately tenfold between the third and seventh decade of life, (ii) the turnover rate of B-cell and T-cell subsets is not substantially altered during healthy aging and despite declining thymic output and naive T-cell numbers with age, the level of peripheral naive T-cell division is not increased in elderly, (iii) in severely lymphocyte-depleted stem-cell transplanted individuals, the cell production rates of deficient B-cell and T-cell subsets are significantly increased, (iv) thymic output and peripheral cell division both contribute substantially to T-cell reconstitution, and (v) in kidney transplantation patients treated with immunosuppressive therapy, low lymphocyte counts do not result in increased cell production rates. These results suggest that in conditions of severe lymphopenia, a homeostatic response enhances the production rate of deficient lymphocyte subsets. At the same time, lymphocyte maintenance during healthy aging does not require substantial alterations in cellular turnover rates. In addition to the human studies, we also performed thymus transplantation and stable-isotope labeling studies in mice to investigate the composition and dynamics of the naive T-cell population in more detail. Our findings suggest that T cells that have recently been produced by the thymus, the recent thymic emigrants, form a subpopulation with distinct kinetics, and are relatively short-lived cells in the naive T-cell pool. Altogether, our fundamental insights into the dynamics of lymphocyte populations will aid the interpretation of past, current, and future investigations in a variety of interventions and diseases, and may form the basis for the development of improved therapeutic strategies that either limit the damage to the lymphocyte compartment, or accelerate immune reconstitution following lymphocyte depletion.

By a balance of cell production and cell loss, B-cell and T-cell populations are generally maintained at a relatively stable size throughout life. Following lymphocyte-depleting therapies however, for example for the treatment of hematological malignancies, reconstitution of some lymphocyte subsets typically takes months to years. Such long-term immunodeficiency results in an increased susceptibility to infectious and malignant diseases and is associated with an increased mortality. Why recovery of these lymphocyte subsets takes so long is poorly understood. To be able to understand the process of immune reconstitution, and how for example advanced aging, and chronic infections affect the lymphocyte compartment, it is essential to first understand how lymphocyte populations are maintained in healthy conditions. The studies in this thesis were focused on investigation of B-cell and T-cell dynamics in healthy conditions and in conditions in which lymphocyte numbers are reduced. By performing in vivo stable isotope labeling studies in healthy volunteers and in lymphopenic stem-cell transplanted and kidney-transplanted patients, we studied how healthy aging, therapeutic lymphocyte depletion, and immunosuppressive therapy affect lymphocyte production and survival rates. In combination with T-cell receptor excision circle analyses and mathematical modeling, we also investigated the relative contributions of thymic output and peripheral cell division to maintenance of the naive T-cell population. The main findings of these studies are that (i) thymic output contributes very little to T-cell production from young adulthood onward, and declines approximately tenfold between the third and seventh decade of life, (ii) the turnover rate of B-cell and T-cell subsets is not substantially altered during healthy aging and despite declining thymic output and naive T-cell numbers with age, the level of peripheral naive T-cell division is not increased in elderly, (iii) in severely lymphocyte-depleted stem-cell transplanted individuals, the cell production rates of deficient B-cell and T-cell subsets are significantly increased, (iv) thymic output and peripheral cell division both contribute substantially to T-cell reconstitution, and (v) in kidney transplantation patients treated with immunosuppressive therapy, low lymphocyte counts do not result in increased cell production rates. These results suggest that in conditions of severe lymphopenia, a homeostatic response enhances the production rate of deficient lymphocyte subsets. At the same time, lymphocyte maintenance during healthy aging does not require substantial alterations in cellular turnover rates. In addition to the human studies, we also performed thymus transplantation and stable-isotope labeling studies in mice to investigate the composition and dynamics of the naive T-cell population in more detail. Our findings suggest that T cells that have recently been produced by the thymus, the recent thymic emigrants, form a subpopulation with distinct kinetics, and are relatively short-lived cells in the naive T-cell pool. Altogether, our fundamental insights into the dynamics of lymphocyte populations will aid the interpretation of past, current, and future investigations in a variety of interventions and diseases, and may form the basis for the development of improved therapeutic strategies that either limit the damage to the lymphocyte compartment, or accelerate immune reconstitution following lymphocyte depletion.

Duisburg, Essen, Univ., Diss., 2013 Die Nabelschnurbluttransplantation hat sich in den letzten 2 Jahrzehnten von einem hochexperimentellen Ansatz zu einer Therapiealternative für Patienten ohne HLA (Human-Leukocyte-Antigen) identischen Spender entwickelt. In dieser Arbeit wurden die Unterschiede in der Immunrekonstitution zwischen einer DUCB (Double Umbilical-Cord-Blood) Kohorte von 42 Patienten, die Stammzellen aus Nabelschnurblut erhielten und einer MUD (Matched-Unrelated Donors) Kohorte von 102 Patienten, die periphere GCSF (Granulocyte Colony-stimulating-factor) -mobilisierte Stammzellen erhielten, prospektiv analysiert. Obwohl in der DUCB Patientenkohorte eine verspätete T Zell Rekonstitution beobachtet werden konnte, die mit einer erhöhten Infektionsrate einherging, traten diese vor allem in den ersten 100 Tagen nach der Transplantation auf. Verglichen mit der MUD Gruppe war die T Zellgesamterholung um 6 Monate verzögert. Dennoch konnten wir keinen Unterschied bezüglich der Rückfallrate, des progressfreien Überlebens (PFS) und des Gesamtüberlebens (OS) zwischen beiden Kohorten feststellen. Die B Zell- und die NK (Natural Killer) Zellerholung war in der DUCB Kohorte ausgeprägter und früher zu beobachten. In der DUCB Kohorte wurde eine signifikant niedrigere cGvHD (chronic Graft-versus-Host Disease) Rate beobachtet, wohingegen die aGvHD (acute Graft-versus-Host Disease) Rate zwischen beiden Gruppen keinen signifikanten Unterschied aufwies. Diese Daten zeigen, dass die Immunrekonstitution in den beobachteten Kohorten über qualitativ unterschiedliche Prozesse erfolgt. Ein verbessertes Verständnis dieser Prozesse könnte einen entscheidenden Fortschritt in der Praxis der allogenen Stammzelltransplantation bewirken. Es wird deutlich, dass die allogene Transplantation mit Nabelschnurblutstammzellen trotz eines erhöhten Infektionsrisikos durch opportunistische Erreger eine ernstzunehmende Alternative darstellt. Die Mechanismen ihrer vielfältigen Vorzüge in der Immunrekonstitution sollten Gegenstand vertiefender Forschung werden.

Interleukin-7 (IL-7) is currently used in a number of clinical trials in humans, including treatment trials for patients with HIV or HCV infection. We would like to draw the attention concerning the impact of IL-7 on the central nervous system (CNS), since most clinical evaluations may not include detailed CNS examination, e.g. Positron Emission Tomography - Computed Tomography (PET-CT) or the evaluation of changes in complex behavior patterns. Increased IL-7-mediated thymic activity demonstrates that viable thymic tissue can be activated in younger individuals; we observed also increased metabolic activity in bone marrow. This has also been observed in clinical studies with individuals who received 30 and 60 μg IL-7 /kg bodyweight leading to increased B-cell progenitors, yet without increased numbers of peripheral B-cells. In summary, our results suggest that IL-7 is able to activate thymic tissue, if functionally receptive. Increased CNS metabolic activity after IL-7 application suggests to monitor the effects of IL-7 for more complex neurological functions and the CNS-metabolic axis in more controlled, comprehensive study settings. It also prepares to consider effects of cytokines on networks of non-lymphoid cells and tissues in patients enrolled in IL-7 trials.