CD28 as a molecular amplifier extending TCR ligation and signaling capabilities.

Evidence has gathered that CD28 costimulation facilitates T cell activation by potentiating TCR intrinsic-signaling. However, the underlying molecular mechanism is largely unknown. Here we show that, by enhancing T cell/APC close contacts, CD28 facilitates TCR signal transduction. Moreover, the signal supplied by CD28 does not lead to increased Zap-70 and Lat phosphorylation, but amplifies PLCgamma1 activation and Ca(2+) response. We provide evidence that the PTK Itk controls the latter function. Our data suggest that CD28 binding to B7 contributes to setting the level of TCR-induced phosphorylated Lat for recruiting signaling complexes, whereas the CD28 signal boosts multiple pathways by facilitating PLCgamma1 activation. These results should provide a conceptual framework for understanding quantitative and qualitative aspects of CD28-mediated costimulation.

CD28 utilizes Vav-1 to enhance TCR-proximal signaling and NF-AT activation.

The mechanism through which CD28 costimulation potentiates TCR-driven gene expression is still not clearly defined. Vav-1, an exchange factor for Rho GTPases thought to regulate, mainly through Rac-1, various signaling components leading to cytokine gene expression, is tyrosine phosphorylated upon CD28 engagement. Here, we provide evidence for a key role of Vav-1 in CD28-mediated signaling. Overexpression of Vav-1 in Jurkat cells in combination with CD28 ligation strongly reduced the concentration of staphylococcus enterotoxin E/MHC required for TCR-induced NF-AT activation. Surprisingly, upon Vav-1 overexpression CD28 ligation sufficed to activate NF-AT in the absence of TCR engagement. This effect was not mediated by overexpression of ZAP-70 nor of SLP-76 but necessitated the intracellular tail of CD28, the intactness of the TCR-proximal signaling cascade, the Src-homology domain 2 (SH2) domain of Vav-1, and SLP-76 phosphorylation, an event which was favored by Vav-1 itself. Cells overexpressing Vav-1 formed lamellipodia and microspikes reminiscent of Rac-1 and Cdc42 activation, respectively, for which the SH2 domain of Vav-1 was dispensable. Together, these data suggest that CD28 engagement activates Vav-1 to boost TCR signals through a synergistic cooperation between Vav-1 and SLP-76 and probably via cortical actin changes to facilitate the organization of a signaling zone.

Plasmodium falciparum AARP1, a giant protein containing repeated motifs rich in asparagine and aspartate residues, is associated with the infected erythrocyte membrane.

During Plasmodium falciparum asexual intraerythrocytic development, the host's cell plasma membrane is modified by the insertion of parasite proteins. One or more of these modifications mediate the cytoadherence of infected erythrocytes to host vascular endothelium. However, these surface antigens can be the target of cytophilic antibodies which promote phagocytosis of the infected erythrocyte. It has been proposed that antibodies directed to epitopes rich in asparagine play an important role in this process, which has promoted efforts to isolate the corresponding gene(s). We describe here P. falciparum asparagine- and aspartate-rich protein 1 (PfAARP1), a new giant (circa 700-kDa) protein associated with the infected erythrocyte membrane which is rich in asparagine and aspartate residues due to the presence of nine blocks of repeats. Topology analysis predicts that PfAARP1 has multiple transmembrane domains and at least five external loops. Human antibodies immunopurified against a sequence composed exclusively of asparagine and aspartate amino acids derived from PfAARP1 label the surface of the infected erythrocyte, demonstrating that such motifs are exposed. Interestingly, external loop 4 of PfAARP1 contains repetitions of these residues, and their possible role as a target of cytophilic antibodies is discussed.

Plasmodium falciparum asparagine and aspartate rich protein 2 is an evolutionary conserved protein whose repeats identify a new family of parasite antigens.

We describe here a new Plasmodium falciparum antigen, asparagine and aspartate rich protein 2 (PfAARP2) of 150 kDa, which is encoded by a unique gene on chromosome 1. PfAARP2 is first expressed 12 h post-invasion and accumulates in trophozoites and schizonts. Immunofluorescence studies indicate that PfAARP2 is translocated into the red blood cell cytoplasm. The central region of Pfaarp2 contains blocks of repetitions encoding asparagine and aspartate residues, which define a new family of related genes dispersed on different chromosomes, and two members of this family have also been identified. Interestingly, the non-repeated N- and C-termini of PfAARP2 display significant similarity to two yeast and human predicted proteins, and its possible function is discussed.