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T cell activation is an essential part of the immune response and requires the activation of the T cell receptor (TCR) and co-receptors, including CD28. After ligand binding, these receptors are internalised into intracellular compartments, sorted for degradation or recycling back to the plasma membrane (PM). Endocytic trafficking plays a central role in the modulation of the immune response, as it regulates intensity and duration of receptor signalling. Much remains to be understood about how these processes are regulated for the TCR and CD28, which are crucial for an effective and sustained T cell activation. In this work, I developed and used a wide range of fluorescence microscopy approaches to investigate the endocytic pathways of TCR and CD28, which are detailed in chapter three. In chapter four, I contributed to decipher the regulation of TCR endocytic trafficking. My data showed, that TCR internalisation is clathrin independent and relies on dynamin for scission of the vesicle from the PM. My results further revealed that after endocytosis, TCR accumulates into an endosomal network which relies on the membrane-organising proteins flotillins. Finally, I contributed to show that flotillins promote sorting of TCR into a fast recycling pathway composed of Rab5 and Rab11-positive endosomes. In chapter five I aimed to understand the role of sorting nexin 9 (SNX9) in CD28 endocytosis. However, the results obtained point towards a different role for SNX9 in the regulation of the spatial organisation of CD28. My data showed that T cell activation leads to the de novo formation of an endosomal network demarked by SNX9. This network is proximal to the immunological synapse and does not extend inside the cell. Furthermore, SNX9 is also present at the PM where it strictly localises within CD28 clusters. I further showed that SNX9 is not required for CD28 clustering, recruitment to the PM or internalisation. Instead, SNX9 is necessary for the stability of CD28 clusters, p

A collection of Pseudomonas syringae and viridiflava isolates was established between 1993 and 2002 from diseased organs sampled from 36 pear, plum and cherry orchards in Belgium . Among the 356 isolates investigated in this study, phytotoxin, siderophore and classical microbiology tests, as well as the genetical methods REP-, ERIC- and BOX-(collectively, rep-) and IS50-PCR, enabled identification to be made of 280 isolates as P. syringae pv. syringae (Pss), 41 isolates as P.syringae pv. morsprunorum (Psm) race 1, 12 isolates as Psm race 2, three isolates as P. viridiflava and 20 isolates as unclassified P. syringae. The rep-PCR methods, particularly BOX-PCR proved to be useful for identifying the Psm race 1 and Ps race 2 isolates. The latter race was frequent on sour cherry in Belgium . Combined genetic results confirmed homogeneities in the pvs avii, and morsprunorum race 1 and race 2 and high diversity in the pv. syringae. In the pv. syringae, homogeneous genetic groups consistently found on the same hosts (pear, cherry or plum) were observed. Pathogenicity on lilac was sometimes variable among Pss isolates from the same genetic group; also,some Psm race 2 and unclassified P. syringae isolates were pathogenic to lilac. In the BOX analyses four patterns included 100% of the toxic lipodepsipeptide(TLP)-producing Pss isolates pathogenic to lilac. Many TLP-producing Pss isolates non-pathogenic to lilac and the TLP-non-producing Pss isolates were classified differently. Pseudomonas syringae isolates that differed from known fruit pathogens were observed in pear, sour cherry and plum orchards in Belgium.