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Most cortical thymocytes die within the thymus. It is proposed here that lethally high intracellular levels of deoxynucleosides are the cause of death. Developing thymocytes appropriately equipped for self-recognition are protected because contact with stromal cells prevents the lethal accumulation of deoxynucleosides.

Naive and memory T-cell subsets differ in their ability to synthesize and respond to a variety o f cytokines in vitro and each subset can produce cytokines that amplify the response of the other subset. The significance of these interactions to antigen responsiveness has, until now, been unclear. In this article Arne Akbar and colleagues point out that both subsets are activated to the same extent by alloantigen and suggest that synergy may be an important event in initiating potent responses against transplanted allografts.

There is now a consensus that T-cell-mediated inflammation and eosinophil activation in the bronchial wall contribute to the pathogenesis of asthma. However, the relationship between these immunopathological mechanisms and the observed physiological aberrations remain unclear. Here, Len Poulter and colleagues identify the links between T-cell-mediated inflammation and bronchial hyperresponsiveness, and propose a hypothesis for asthma pathogenesis in which the combination of immunological and physiological abnormalities may result in the promotion of disease. Furthermore, they suggest that an integral factor in the prevention of this process is the regulation of bronchial T-cell reactivity by a population of immunosuppressive macrophages.

One of the central questions in T-cell immunity is how the generation of T-cell memory may proceed simultaneously with the maintenance of T-cell homeostasis. In this article, we present a hypothesis that the regulation of bcl-2 gene expression within the activated CD45RO+ T-cell population may be one of the key factors in the balance between death and survival of these activated mature T cells, both in vitro and in vivo.

As it has become clear that HIV causes a progressive breakdown in immune competence that can be precisely measured, the introduction of diagnostic tests in placebo-controlled clinical trials has taken on a new importance. The task for clinical immunologists, namely to standardize these techniques through international collaboration using current understanding of patho genesis, was the subject of a recent workshop.

Bone marrow transplantation (BMT) is undertaken in a heroic attempt to increase the cute rate in malignancy and to treat deadly metabollic disorders. After ablative therapy, patients are transfused with their own or with HLA matched or haploidentical bone marrow. The inoculum cells or mature T cells which cause graft versus host disease. Meetings such as the recent First International Workshop on Bone Marrow are important as a forum for a review of current developments, advice about the selection of techniques - many of them exquisitely ingenious and efficient - and for a discussion of the exact role of bone marrow purging in the complex process of patient management.