No abstract

The dual specificity of thymus-derived lymphocytes (T cells) for structures coded by the major histocompatibility gene complex (MHC) 1 and for foreign antigens (X) appears to be a general finding in mice and is likely to be a more universal phenomenon in higher vertebrates. Thus, all T-cell functions that have been tested in mice i.e., T cells involved in nonlytic helper, delayed type hypersensitivity, and macrophage activation functions are specific for the murine MHC (H-2) coded structure mapping to the I region (1-5), whereas cytotoxic T-cell activity is specific for H-2K or D (6-11). Similarly, cytotoxicity appears to be MHC-restricted in the rat (12), in humans (13), and in chickens (14). This restriction of T cells by MHC determinants contrasts with the apparently complete absence of H-2 restrictedness of B-cell functions or of their antibody products.Although the phenomenology is clear and well accepted, the explanation of it is controversial and therefore many hypotheses and speculations have attempted to catch the elusive nature of the T-cell receptor; because it is generally felt that this dual specificity of T cells reflects and may therefore reveal unique properties of the T-cell receptor(s). Two models of associative T-cell recognition have been proposed: first the dual recognition model (1-9, 14, 15) where T cells recognize two distinct antigenic entities i.e., self-H-2 structures and foreign antigen X with two separate receptors. Second, the single receptor model, which proposes that T cells possess one single receptor specificity that recognizes a neoantigenic determinant (NAD) formed either by a complex of self and

In this paper we study how a benevolent government that cannot commit to future policy should trade off the costs and benefits of public expenditure. We characterize and solve for Markov-perfect equilibria of the dynamic game between successive governments. The characterization consists of an inter-temporal first-order condition (a "generalized Euler equation") for the government, and we use it both to gain insight into the nature of the equilibrium and as a basis for computations. For a calibrated economy, we find that when the only tax base available to the government is capital income-an inelastic source of funds at any point in time-the government still refrains from taxing at confiscatory rates. We also find that when the only tax base is labour income the Markov equilibrium features less public expenditure and lower tax rates than the Ramsey equilibrium. Copyright © 2008 The Review of Economic Studies Limited.

Spirorchiid trematodes are implicated as an important cause of stranding and mortality in sea turtles worldwide. However, the impact of these parasites on sea turtle health is poorly understood due to biases in study populations and limited or missing data for some host species and regions, including the southeastern United States. We examined necropsy findings and parasitological data from 89 loggerhead Caretta caretta and 59 green turtles Chelonia mydas that were found dead or moribund (i.e. stranded) in Florida (USA) and evaluated the role of spirorchiidiasis in the cause of death. High prevalence of infection in the stranding population was observed, and most infections were regarded as incidental to the cause of death. Spirorchiidiasis was causal or contributory to death in some cases; however, notable host injury and/or large numbers of parasites were observed in some animals, including nutritionally robust turtles, with no apparent relationship to cause of death. New spirorchiid species records for the region were documented and identified genera included Neospirorchis, Hapalotrema, Carettacola, and Learedius. Parasites inhabited and were associated with injury and inflammation in a variety of anatomic locations, including large arteries, the central nervous system, endocrine organs, and the gastrointestinal tract. These findings provide essential information on the diversity of spirorchiids found in Florida sea turtles, as well as prevalence of infection and the spectrum of associated pathological lesions. Several areas of needed study are identified with regard to potential health implications in the turtle host, and findings caution against over-interpretation in individual cases.

Green turtle fibropapillomatosis (GTFP), characterized by multiple benign fibroepithelial tumors on the skin and eyes, has become a growing threat to green turtle Chelonja mydas populations worldwide. The cause of GTFP 1s unknown, but a viral etiology is suspected. This study investigated whether GTFP could be experimentally transmitted to young captive-reared green turtles using cell-free fibropapilloma extracts prepared from free-ranging turtles with spontaneous disease Turtles raised from eggs collected from 4 separate clutches in the wild were assigned to 4 expenmental groups and 1 control group. For each experiment a crude homogenate (33 % w/v) was prepared from fibropapillomas removed from a free-ranging turtle with spontaneous disease. The crude tumor homogenates were freeze-thawed and centrifuged to yield cell-free extracts that were used (both filtered and unaltered) for inoculation. Recipients were inoculated by intradermal injection or by scarification; control turtles were not treated but \yere housed with treated turtles. Fibropapillomas developed in all 12 turtles receiving 3 of the 4 tumor extracts, and were first detected between 15 and 43 wk post inoculation. Both filtered and unfiltered tumor extracts successfully induced tumor development. During the 10 and 12 mo monitoring periods. fibropapillomas did not develop in control turtles or in any turtles inoculated with the fourth tumor extract. Although 2 sets of experiments were performed 8 wk apart, most of the tumors in both sets became evident simultaneously after water temperatures rose Experimental tumors were h~stologically i n d~s -tinguishable from spontaneous fibropapillomas found In free-living turtles but lacked evidence of endoparasites. Scattered foci of epidermal degeneration were found in most sections of experimentally induced fibropapillomas and within some sections taken from donor turtles. Electron rnicroscopy revealed virus-like particles conforming in size, morphology, and intranuclear location with herpesvirus. Negativestaining electron microscopy of transmission-positive tumor extracts failed to demonstrate intact virus particles. This study demonstrates that the etiology of GTFP is an infectious filterable subcellular agent. The herpesvirus identified in this study is 1 possible candidate for the etiology of GTFP.