These guidelines are a consensus work of a considerable number of members of the immunology and flow cytometry community. They provide the theory and key practical aspects of flow cytometry enabling immunologists to avoid the common errors that often undermine immunological data. Notably, there are comprehensive sections of all major immune cell types with helpful Tables detailing phenotypes in murine and human cells. The latest flow cytometry techniques and applications are also described, featuring examples of the data that can be generated and, importantly, how the data can be analysed. Furthermore, there are sections detailing tips, tricks and pitfalls to avoid, all written and peer‐reviewed by leading experts in the field, making this an essential research companion.

The loss of naïve T cells is a hallmark of immune aging. Although thymic involution is a primary driver of this naïve T cell loss, less is known about the contribution of other mechanisms to the depletion of naïve T cells in aging primates. We examined the role of homeostatic cycling and proliferative expansion in different T cell subsets of aging rhesus macaques (RM). BrdU incorporation and the expression of the G1-M marker Ki-67 were elevated in peripheral naïve CD4 and even more markedly in the naïve CD8 T cells of old, but not young adult, RM. Proliferating naïve cells did not accumulate in old animals. Rather, the relative size of the naïve CD8 T cell compartment correlated inversely to its proliferation rate. Likewise, T cell receptor diversity decreased in individuals with elevated naïve CD8 T cell proliferation. This apparent contradiction was explained by a significant increase in turnover concomitant with the naïve pool loss. The turnover increased exponentially when the naïve CD8 T cell pool decreased below 4% of total blood CD8 cells. These results link the shrinking naïve T cell pool with a dramatic increase in homeostatic turnover, which has the potential to exacerbate the progressive exhaustion of the naïve pool and constrict the T cell repertoire. Thus, homeostatic T cell proliferation exhibits temporal antagonistic pleiotropy, being beneficial to T cell maintenance in adulthood but detrimental to the long-term T cell maintenance in aging individuals.aging ͉ CD8 ͉ homeostasis T he immune system undergoes dramatic age-related changes in both structure and function. Of these, the best investigated and the most pronounced are the changes affecting aging T cells. Age-related changes were seen in T cell subset representation and T cell diversity in humans (1-3) and experimental animals (4, 5) and have been associated with higher mortality in the aging humans (6, 7). Moreover, impaired T cell activation has been documented in senescent rodent and human T cells (8)(9)(10)(11). Although the molecular lesions in T cell activation have been thoroughly studied, the picture of the age-related dysregulation of T cell populations is still emerging.Optimal protection against new or evolving pathogens requires a reserve of naïve T cells. However, naïve T cell production is reduced in old age because of the involution of the thymus. The maintenance of the naïve T cell compartment is further challenged by the lifelong consumption of naïve cells that respond to acute and persistent infections and become memory T cells. With age, the balance between naïve and antigenselected memory cells changes in favor of the latter, reducing the diversity of T cell receptors (TCRs). In fact, the aging naïve T cell population is reduced in both relative and absolute terms, suggesting that homeostatic mechanisms that maintain the size and the clonal diversity of the T cell repertoire progressively break down (12)(13)(14).T cells can also be replenished by extrathymic mechanisms, which rely on homeostatic proliferation in the absenc...

The third edition of Flow Cytometry Guidelines provides the key aspects to consider when performing flow cytometry experiments and includes comprehensive sections describing phenotypes and functional assays of all major human and murine immune cell subsets. Notably, the Guidelines contain helpful tables highlighting phenotypes and key differences between human and murine cells. Another useful feature of this edition is the flow cytometry analysis of clinical samples with examples of flow cytometry applications in the context of autoimmune diseases, cancers as well as acute and chronic infectious diseases. Furthermore, there are sections detailing tips, tricks and pitfalls to avoid. All sections are written and peer-reviewed by leading flow cytometry experts and immunologists, making this edition an essential and state-of-the-art handbook for basic and clinical researchers.

In contrast to many other virus infections, primary cytomegalovirus (CMV) infection does not fully protect against reinfection. Accordingly, clinical data have revealed a coexistence of multiple human CMV variants/ strains in individual patients. Notably, the phenomenon of multiple infection was found to correlate with increased virus load and severity of CMV disease. Although of obvious medical relevance, the mechanism underlying this correlation is unknown. A weak immune response in an individual could be responsible for a more severe disease and for multiple infections. Alternatively, synergistic contributions of variants that differ in their biological properties can lead to qualitative changes in viral fitness by direct interactions such as genetic recombination or functional complementation within coinfected host cells. We have addressed this important question paradigmatically with the murine model by differently designed combinations of two viruses employed for experimental coinfection of mice. Specifically, a murine cytomegalovirus (MCMV) mutant expressing Cre recombinase was combined for coinfection with a mutant carrying Cre-inducible green fluorescent protein gene, and attenuated mutants were combined for coinfection with wild-type virus followed by two-color in situ hybridization studies visualizing the replication of the two viruses in infected host organs. These different approaches concurred in the conclusion that coinfection of host cells is more frequent than statistically predicted and that this coinfection alters virus fitness by functional trans-complementation rather than by genetic recombination. The reported findings make a major contribution to our molecular understanding of enhanced CMV pathogenicity in the multiply infected host.

Aging is usually accompanied by diminished immune protection upon infection or vaccination. While aging results in well-characterized changes in the T-cell compartment of long-lived, outbred, and pathogen-exposed organisms, their relevance for primary antigen responses remain unclear. Therefore, it remains unclear whether and to what extent the loss of naïve T-cells, their partial replacement by oligoclonal memory populations, and the consequent constriction of T-cell receptor (TCR) repertoire, limit the antigen responses in aging primates.