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This thesis investigates lymphocyte dynamics in a more natural mouse model (wildling mice) and translates these findings to the human situation. It re-evaluates known rates in standard mice and explores unaddressed questions.
It has been suggested that replacement rates of immune cells are influenced by the microbial status of mice. We found that neither the replacement rates of natural killer cells, nor those of naive T cells are influenced by the microbial status. Memory T cells on the other side showed increased replacement rates in mice with a more natural microbiome (wildling mice). Naive T cell maintenance in wildling and standard laboratory mice are comparable but show significant differences to human naive T cell maintenance.

We were interested in the dynamics of memory T cells residing in tissues. Microbial exposure leads to a more natural immune phenotype in mice, resembling the human situation, with more abundant memory T cells in tissue. Tissues of standard laboratory mice contain few memory T cells. In wildling mice, we found consistently shorter lifespans of circulating CD69- memory T cells when compared to CD69+ tissue resident memory T (TRM) cells. We wondered whether the same observation could be made in humans and studied memory T cells from skin, adipose tissue and bone marrow. The difference between CD69- and CD69+ memory T cells was generally less pronounced in humans. However, in both humans and mice, bone-marrow derived CD69+ memory T cells had the slowest replacement rates of all memory T cells that we studied.

Focusing on T cell dynamics in disease, we studied how CCR5 inhibition influences T cell dynamics in the context of HIV-1 infection. CCR5 inhibition by maraviroc was studied in people with suboptimal immune reconstitution despite viral suppression (immunological non-responders, INRs). We compared T cell replacement rates of INR to those in HIV-infected people with successful immune reconstitution (immunological responders, IRs), and found faster T cell replacement rates in INRs. INRs receiving a maraviroc add-on therapy, had T cell replacement rates similar to those of IRs. Surprisingly, maraviroc did not influence T cell replacement rates in mice, suggesting that the effect of maraviroc on T cell replacement rates in INRs is not merely due to blocking CCR5 signalling on T cells.

We explored a potential alternative for deuterium labelling in mice. The use of 5-Iodo-2’-deoxyuridine (IdU) could offer simultaneous analysis of cell markers and kinetics by CyTOF®. This would offer the benefit to assess label incorporation on a single cell level. Unfortunately, IdU turned out to increase cell numbers in spleen as well as memory T cell replacement rates, making it an unreliable tool for quantifying T cell replacement rates.

This thesis provides several examples where animal models provided valuable information that could hardly or not at all be obtained from humans. The discrepancies between human and murine immune dynamics found in parts of this thesis highlight, however, some fundamental differences between murine and human immune cell maintenance. While animal models still hold potential, their critical discussion is therefore crucial.

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