Antonio A. Freitas

Homeostasis of peripheral CD4+ T cells: IL-2R alpha and IL-2 shape a population of regulatory cells that controls CD4+ T cell numbers.

We show that the lymphoid hyperplasia observed in IL-2Rα- and IL-2-deficient mice is due to the lack of a population of regulatory cells essential for CD4 T cell homeostasis. In chimeras reconstituted with bone marrow cells from IL-2Rα-deficient donors, restitution of a population of CD25+CD4+ T cells prevents the chaotic accumulation of lymphoid cells, and rescues the mice from autoimmune disease and death. The reintroduction of IL-2-producing cells in IL-2-deficient chimeras establishes a population of CD25+CD4+ T cells, and restores the peripheral lymphoid compartments to normal. The CD25+CD4+ T cells regulated selectively the number of naive CD4+ T cells transferred into T cell-deficient hosts. The CD25+CD4+/naive CD4 T cell ratio and the sequence of cell transfer determines the homeostatic plateau of CD4+ T cells. Overall, our findings demonstrate that IL-2Rα is an absolute requirement for the development of the regulatory CD25+CD4+ T cells that control peripheral CD4 T cell homeostasis, while IL-2 is required for establishing a sizeable population of these cells in the peripheral pools.

TLR-Activated B Cells Suppress T Cell-Mediated Autoimmunity

TLR sense microbial infections, and control activation of immune responses. Dendritic cells, macrophages, and B lymphocytes express TLR and the TLR-signaling adaptor protein MyD88. The impact of TLR-activated B cells on T cell-mediated inflammation is unknown. In this study, we have used mice carrying B cell-restricted deficiencies in MyD88 or in distinct TLR to examine the impact of TLR-activated B cells on a T cell-mediated autoimmune disease, experimental autoimmune encephalomyelitis (EAE). We demonstrate that TLR-signaling in B cells suppresses inflammatory T cell responses (both Th1 and Th17), and stimulates recovery from EAE. Only certain TLR are required on B cells for resolution of EAE, and these are dispensable for disease initiation, indicating that a category of TLR agonists preferentially triggers a suppressive function in B cells and thereby limits autoimmune disease. The TLR agonists controlling the regulatory function of B cells are provided by components of Mycobacterium tuberculosis present in the adjuvant. Thus, MyD88 signaling in B cells antagonizes MyD88 signaling in other cells, which drives differentiation of Th17 cells and is required for induction of EAE. Altogether, our data indicate that B cells link recognition of microbial products via TLR to suppression of a T cell-mediated autoimmune disease.

Lymphocyte Population Kinetics in the Mouse

In a normal dynamic equilibrium, at least half of the peripheral T-cell pool is constituted by lymphocytes which have divided 24-48 h previously, and are therefore rapidly renewed. The renewal of peripheral T cells occurs partly by influx of cells from the thymus and, more importantly, by cell division at the periphery. The cyclic pattern of decay observed for T cells after HU treatment suggests the presence of progenitor-descendent relationships within the peripheral T-cell pool. Peripheral progenitors must contain both cycling and non-cycling cells to account for cell recovery after HU administration in ATx mice. T-cell production at the periphery involves both organized (spleen or lymph nodes) as well as non-organized lymphoid tissue (GALT). The latter may in fact provide the major contribution. Expansion of mature T lymphocytes contributes to clonal persistence at the periphery and to the choice of T-cell repertoires. The importance of post-thymic selection of T-cell repertoires is suggested by the considerable expansion potential revealed by peripheral T cells.

Antiapoptotic Mcl-1 is critical for the survival and niche-filling capacity of Foxp3 + regulatory T cells

Foxp3+ regulatory T (Treg) cells are a crucial immunosuppressive population of CD4+ T cells, yet the homeostatic processes and survival programs that maintain the Treg cell pool are poorly understood. Here we report that peripheral Treg cells markedly alter their proliferative and apoptotic rates to rapidly restore numerical deficit through an interleukin 2–dependent and costimulation-dependent process. By contrast, excess Treg cells are removed by attrition, dependent on the Bim-initiated Bak- and Bax-dependent intrinsic apoptotic pathway. The antiapoptotic proteins Bcl-xL and Bcl-2 were dispensable for survival of Treg cells, whereas Mcl-1 was critical for survival of Treg cells, and the loss of this antiapoptotic protein caused fatal autoimmunity. Together, these data define the active processes by which Treg cells maintain homeostasis via critical survival pathways.

Indexation as a Novel Mechanism of Lymphocyte Homeostasis: The Number of CD4+CD25+ Regulatory T Cells Is Indexed to the Number of IL-2-Producing Cells

To fulfill its mission, the immune system must maintain a complete set of different cellular subpopulations that play specific roles in immune responses. We have investigated the mechanisms regulating CD4+CD25+ regulatory T (Treg) cell homeostasis. We show that the expression of the high-affinity IL-2Rα endows these cells with the capacity to explore the IL-2 resource, ensuring their presence while keeping their number tied to the number of CD4+ T cells that produce IL-2. We show that such a homeostatic mechanism allows the increased expansion of T cells without causing disease. The indexing of Treg cells to the number of activated IL-2-producing cells may constitute a feedback mechanism that controls T cell expansion during immune responses, thus preventing autoimmune or lymphoproliferative diseases. The present study highlights that maintenance of proportions between different lymphocyte subsets may also be critical for the immune system and are under strict homeostatic control.

Peripheral T cell survival

T cell survival in the periphery is an active process, depending on continuous TCR engagement by peptide-MHC complexes and/or response to environmental cytokines. Naive T cells require interactions with the MHC restricting element. The survival requirements of memory T cells are as yet insufficiently characterized, but MHC-restricted interactions are not necessary.

B-cell homeostasis, competition, resources, and positive selection by self-antigens

Summary:  In adult mice, the number of B lymphocytes remains constant under homeostatic control, in spite of the fact that B cells are produced continuously in numbers that largely exceed the number required to replenish the peripheral pools. It follows that each newly formed lymphocyte can only persist if another lymphocyte dies. In an immune system where the total number of cells is limited, cell survival is no longer a passive phenomenon but rather a continuous active process where each lymphocyte must compete with other lymphocytes to survive. Consequently, the number and the life expectancy of a B‐cell clone vary according to the presence or absence of competitor populations. This process of lymphocyte competition is likely controlled by a common need for resources that are in limited supply. The number of peripheral B‐cells varies according to the availability of B‐cell receptor (BCR) ligands. Indeed, it is possible to modify steady‐state B‐cell numbers by antigen manipulation. Moreover, conventional self‐reactive B cells can undergo positive selection. We showed that the fate of a self‐reactive B cell is determined by the quantity of self‐antigens, the number of antigen‐specific receptors engaged, and its overall antigen‐binding avidity rather than the affinity of individual BCRs.

Functional Analysis via Standardized Whole-Blood Stimulation Systems Defines the Boundaries of a Healthy Immune Response to Complex Stimuli

Standardization of immunophenotyping procedures has become a high priority. We have developed a suite of whole-blood, syringe-based assay systems that can be used to reproducibly assess induced innate or adaptive immune responses. By eliminating preanalytical errors associated with immune monitoring, we have defined the protein signatures induced by (1) medically relevant bacteria, fungi, and viruses; (2) agonists specific for defined host sensors; (3) clinically employed cytokines; and (4) activators of T cell immunity. Our results provide an initial assessment of healthy donor reference values for induced cytokines and chemokines and we report the failure to release interleukin-1α as a common immunological phenotype. The observed naturally occurring variation of the immune response may help to explain differential susceptibility to disease or response to therapeutic intervention. The implementation of a general solution for assessment of functional immune responses will help support harmonization of clinical studies and data sharing.

Quorum-Sensing in CD4(+) T Cell Homeostasis: A Hypothesis and a Model.

Homeostasis of lymphocyte numbers is believed to be due to competition between cellular populations for a common niche of restricted size, defined by the combination of interactions and trophic factors required for cell survival. Here we propose a new mechanism: homeostasis of lymphocyte numbers could also be achieved by the ability of lymphocytes to perceive the density of their own populations. Such a mechanism would be reminiscent of the primordial quorum-sensing systems used by bacteria, in which some bacteria sense the accumulation of bacterial metabolites secreted by other elements of the population, allowing them to “count” the number of cells present and adapt their growth accordingly. We propose that homeostasis of CD4+ T cell numbers may occur via a quorum-sensing-like mechanism, where IL-2 is produced by activated CD4+ T cells and sensed by a population of CD4+ Treg cells that expresses the high-affinity IL-2Rα-chain and can regulate the number of activated IL-2-producing CD4+ T cells and the total CD4+ T cell population. In other words, CD4+ T cell populations can restrain their growth by monitoring the number of activated cells, thus preventing uncontrolled lymphocyte proliferation during immune responses. We hypothesize that malfunction of this quorum-sensing mechanism may lead to uncontrolled T cell activation and autoimmunity. Finally, we present a mathematical model that describes the key role of IL-2 and quorum-sensing mechanisms in CD4+ T cell homeostasis during an immune response.

Impaired regeneration of the peripheral B cell repertoire from bone marrow following lymphopenia in old mice

Aging is associated with a decreased production of B cells by the bone marrow and an increased life‐span of peripheral B cells. To determine whether the decreased bone marrow B cell production is linked to the increased life‐span of B cells in old mice, B cell regeneration following lymphopenia was studied in young and old mice. The rate of bone marrow pre‐B cell and of splenic B cell regeneration is slower in irradiated, old compared to irradiated, young recipients of young, congeneic bone marrow. This finding reflects an age‐associated defect in the bone marrow microenvironment. As the bone marrow is the only source of a diverse population of B cells, we measured the diversity of the splenic B cell repertoire regenerated following drug‐induced lymphopenia in old and young mice. The heterogeneity of mRNA size from IgH complementarity determining region 3 (CDR3) was more restricted in splenic B cells from old compared to young mice providing additional evidence for an age‐associated impairment in B cell production by the bone marrow.