Abstract-CD40 is a 48-kDa phosphorylated transmembrane glycoprotein belonging to the TNF receptor superfamily.CD40 has been demonstrated on a range of cell types, and it has an important role in adaptive immunity and inflammation. CD40 has recently been described on platelets but platelet activation by CD40 has not been described.In the present study, we use flow cytometry and immunoblotting to confirm that platelets constitutively express surface CD40. CD40 mRNA was undetectable, suggesting that the protein is synthesized early in platelet differentiation by megakaryocytes. Ligation of platelet CD40 with recombinant soluble CD40L trimer (sCD40LT) caused increased platelet CD62P expression, ␣-granule and dense granule release, and the classical morphological changes associated with platelet activation. CD40 ligation also caused ␤ 3 integrin activation, although this was not accompanied by platelet aggregation. These actions were abrogated by the CD40L blocking antibody TRAP-1 and the CD40 blocking antibodies M2 and M3, showing that activation was mediated by CD40L binding to platelet CD40. ␤ 3 integrin blockade with eptifibatide had no effect, indicating that outside-in signaling via ␣ IIb ␤ 3 was not contributing to these CD40-mediated effects. CD40 ligation led to enhanced platelet-leukocyte adhesion, which is important in the recruitment of leukocytes to sites of thrombosis or inflammation. Our results support a role for CD40-mediated platelet activation in thrombosis, inflammation, and atherosclerosis.

The skin is an important immunological organ with an outer protective layer, the stratum corneum forming a barrier between the skin-associated lymphoid tissue and the environment. We show that gently removing the stratum corneum with adhesive tape permits potent epicutaneous immunization to protein antigens. IL-4 secretion by T cells from draining lymph nodes and high levels of specific IgE and IgG1 with no IgG2a showed that the immune responses induced following epicutaneous antigen exposure are strongly Th2 biased. Similar responses were obtained with different antigens and mouse strains. In contrast, subcutaneous immunization with antigen delivery into the dermis was less potent and gave predominantly Th1 responses. Removal of the stratum corneum increased expression of MHC class II, CD86, CD40, CD54 and CD11c on Langerhans cells, but did not cause them to migrate. Rapid migration from epidermis to draining lymph node was obtained, however, by exposure to antigen after removal of the stratum corneum, suggesting that maturation and migration of Langerhans cells are independently regulated events. These results suggest that antigen presentation by Langerhans cells gives predominantly Th2 responses. This may provide an explanation for allergic sensitization to some antigens. It may also be a useful non-invasive, non-adjuvant-dependent method of vaccination.

We consider the lifetime of a T cell clonotype, the set of T cells with the same T cell receptor, from its thymic origin to its extinction in a multiclonal repertoire. Using published estimates of total cell numbers and thymic production rates, we calculate the mean number of cells per TCR clonotype, and the total number of clonotypes, in mice and humans. When there is little peripheral division, as in a mouse, the number of cells per clonotype is small and governed by the number of cells with identical TCR that exit the thymus. In humans, peripheral division is important and a clonotype may survive for decades, during which it expands to comprise many cells. We therefore devise and analyse a computational model of homeostasis of a multiclonal population. Each T cell in the model competes for self pMHC stimuli, cells of any one clonotype only recognising a small fraction of the many subsets of stimuli. A constant mean total number of cells is maintained by a balance between cell division and death, and a stable number of clonotypes by a balance between thymic production of new clonotypes and extinction of existing ones. The number of distinct clonotypes in a human body may be smaller than the total number of naive T cells by only one order of magnitude.

What are the rules that govern a naive T cell's prospects for survival or division after export from the thymus into the periphery? To help address these questions, we combine data from existing studies with robust mathematical models to estimate the absolute contributions of thymopoiesis, peripheral division, and loss or differentiation to the human naive CD4 ؉ T-cell pool between the ages of 0 and 20 years. Despite their decline in frequency in the blood, total body numbers of naive CD4 ؉ T cells increase throughout childhood and early adulthood. Our analysis shows that postthymic proliferation contributes more than double the number of cells entering the pool each day from the thymus. This ratio is preserved with age; as the thymus involutes, the average time between naive T-cell divisions in the periphery lengthens. We also show that the expected residence IntroductionAn adult human has a population of approximately 10 11 naive T cells circulating in the peripheral lymphoid organs and blood. From early in development, this population is generated and sustained by thymic export and division on the periphery, and is estimated to comprise at least 10 8 different T-cell receptor specificities, 1 providing a broad spectrum of protection in a diverse pathogen environment.The rate of export of naive T cells from the thymus declines substantially with age in healthy persons, 2 but estimates of the total number of cells exported from the thymus over a person's lifetime are still approximately 10-fold greater than the total number of naive T cells in an adult at any one time; an estimate of daily thymic output based on the study of Steinmann et al 2 integrated over 80 years is 5 ϫ 10 12 cells. Furthermore, at least a subset of naive cells continues to divide slowly after release from the thymus into the periphery. These 2 observations imply that turnover and replacement occurs in the naive T-cell pool. What are the rules that govern a circulating naive cell's prospects for survival and proliferation? Do these rules change as we age and, if so, how? Identifying these rules requires a combination of experimental approaches and mathematical models, and will provide an essential background for understanding the dynamics of the T-cell pool when it is dysregulated-for example, during the reconstitution of the T-cell pools after medical interventions that induce lymphopenia, or after antiretroviral therapy in HIV infection. 3 As a step toward answering these questions, here we quantify the contributions of proliferation, loss, and thymic input to the development of the healthy naive T-cell compartment. We focus on naive CD4 ϩ T-cell dynamics in persons up to age 20. The youngest age groups might be expected to have the most dynamic T-cell populations because rates of thymic export are highest and physiologic growth, in particular growth of blood volume and lymphoid tissue, is continuously altering the environment in which the T cells circulate and encounter homeostatic signals.Currently, the most direct methods for measurin...

Epicutaneous exposure to peanut protein can prevent induction of oral tolerance, and may even modify existing tolerance to peanut. Epidermal exposure to protein allergens selectively drives Th2-type responses, and as such may promote sensitization to food proteins upon gastrointestinal exposure.