Discovering the origins of immunological competence

The evolving role of T-bet in resistance to infection

The identification of T-bet as a key transcription factor associated with the development of IFNγ-producing CD4⁺ T cells predicted a crucial role for T-bet in cell-mediated immunity and in resistance to many intracellular infections. This idea was reinforced by initial reports showing that T-bet-deficient mice were more susceptible to pathogens that survived within the lysosomal system of macrophages. However, subsequent studies revealed IFNγ-dependent, T-bet-independent pathways of resistance to diverse classes of microorganisms that occupy other intracellular niches. Consequently, a more complex picture has emerged of how T-bet and the related transcription factor eomesodermin (EOMES) coordinate many facets of the immune response to bona fide pathogens as well as commensals. This article provides an overview of the discovery and evolutionary relationship between T-bet and EOMES and highlights the studies that have uncovered broader functions of T-bet in innate and adaptive immunity and in the development of the effector and memory T cell populations that mediate long-term resistance to infection.

Thymic function in juvenile idiopathic arthritis

OBJECTIVE

Thymic function declines exponentially with age. Impaired thymic function has been associated with autoimmune disease in adults but has never been formally assessed in childhood autoimmunity. Therefore, thymic function in children with the autoimmune disease juvenile idiopathic arthritis (JIA) was determined.

METHODS

Thymic function was measured in 70 children and young adults with JIA (age range 2.1-30.8 (median 10.4)) and 110 healthy age-matched controls using four independent assays. T cell receptor excision circles (WBLogTREC/ml) and the proportion of CD4(+) CD45RA(+)CD31(+) T cells (representing recent thymic emigrants; %RTEs) were quantified and intrathymic proliferation measured by calculating the alphaTREC/SigmabetaTREC ratio. Lastly, regulatory T cells (T(Reg)) of thymic origin (CD4(+)FOXP3(+)) were quantified in peripheral blood to assess the ability of the thymus in JIA to generate this T cell subset.

RESULTS

Thymic function was equivalent by all four parameters in JIA when compared with the control population. Furthermore, there was no consistent effect of JIA subtype on thymic function, although intrathymic proliferation was higher in the small rheumatoid factor (RF)(+) polyarticular group. There were no significant effects of disease-modifying antirheumatic drugs (DMARDs) or oral corticosteroids on thymic function, although those with the worst prognostic ILAR (International League of Associations for Rheumatology) subtypes were also those most likely to be on a DMARD.

CONCLUSIONS

It is demonstrated that children and young adults with JIA, unlike adults with autoimmune diseases, have thymic function that is comparable with that of healthy controls. The varied pathologies represented by the term "JIA" suggest this observation may not be disease specific and raises interesting questions about the aetiology of thymic impairment in adult autoimmunity.

A brief history of CD8 T cells

CD8 T cells comprise a powerful branch of the adaptive immune system, yet were not formally recognized until long after the discovery of antibody. CD8 T cells contribute to the eradication of intracellular infections and to the control of many chronic infections. There is tempered optimism that CD8 T cell memory elicited via vaccination may hold the key to manufacturing protective immunity against pathogens that cause chronic infections in humans. The specificity and destructive capabilities of CD8 T cells may also be harnessed for the eradication of tumors. However, CD8 T cells also contribute to a variety of clinical difficulties such as immune mediated pathology, rejection of organ transplants, and autoimmunity. Clearly, learning how to safely generate protective long-lived CD8 T cell memory and how to control or eliminate specific unwanted responses could deliver substantial clinical rewards, and there is a great need for continued research on the biology and therapeutic potential of these cells. Herein, we recount the historical developments leading to the discovery of CD8 T cells, highlight a few of the important discoveries that have followed, and discuss some of the critical issues on the horizon.

The role of the transcription factor Foxp3 in the development of regulatory T cells

Early studies of mice subjected to neonatal thymectomy and analyses of adoptive T-cell transfer into lymphopenic hosts led to the identification of a specialized subset of regulatory CD4+ T cells capable of suppressing various manifestations of autoimmunity. Recently, a combination of genetic, molecular, and traditional cellular approaches provided novel powerful means to investigate the biology of these cells. Here, we review earlier and current work from our laboratory, establishing a dedicated function for the transcription factor Foxp3 in the process of regulatory T-cell lineage commitment and a role for TCR- and cytokine-mediated signals in regulation of Foxp3 expression.

Toward tolerance to human organ transplants: a few additional corollaries and questions1

Several component mechanisms of "central" (thymic) and "peripheral" tolerance in human organ transplant recipients are briefly discussed, in opposition to a more confined view limited to clonal depletion and exhaustion as proposed by Starzl (Transplantation 2004; 77(6): 926). Attention is directed to more than 40 years of experimental work in adult animal species dealing with immunoregulation. This work is in contradistinction to the simpler depletion/exhaustion explanation of Starzl (and Zinkernagel) regarding the potential of human organ transplant recipients to be significantly (or totally) withdrawn from continuous immunosuppression. Other observations are included touching on the roles of certain viral infections, the thymus, and the bone marrow in arriving at a state of "alloimmune homeostasis" in humans.

The thymus and negative selection

Maintenance of tolerance to self antigens is presumed to reflect a combination of central and peripheral tolerance. For T cells, central tolerance occurs during early T cell development in the thymus and causes cells with strong reactivity to self antigens to be destroyed in situ (negative selection). Here, we summarize evidence that negative selection can occur in the thymic medulla and affects a population of semimature HSA+ T cells. The influence of costimulatory molecules, Fas and cytokines on negative selection is discussed.

Accessing complexity: the dynamics of virus-specific T cell responses

The cellular dynamics of the immune system are complex and difficult to measure. Access to this problematic area has been greatly enhanced by the recent development of tetrameric complexes of MHC class I glycoprotein + peptide (tetramers) for the direct staining of freshly isolated, antigen-specific CD8(+ )T cells. Analysis to date with both naturally acquired and experimentally induced infections has established that the numbers of virus-specific CD8(+) T cells present during both the acute and memory phases of the host response are more than tenfold in excess of previously suspected values. The levels are such that the virus-specific CD8(+) set is readily detected in the human peripheral blood lymphocyte compartment, particularly during persistent infections. Experimentally, it is now possible to measure the extent of cycling for tetramer (+)CD8(+) T cells during the acute and memory phases of the host response to viruses. Dissection of the phenotypic, functional, and molecular diversity of CD8(+) T cell populations has been greatly facilitated. It is hoped it will also soon be possible to analyze CD4(+) T cell populations in this way. Though these are early days and there is an enormous amount to be done, our perceptions of the shape of virus-specific cell-mediated immunity are changing rapidly.