Citations

Abstract The NK cell-activating receptor NKG2D plays a critical role in the destruction of malignant cells, but many of the cell-signaling mechanisms governing NKG2D-mediated cellular cytotoxicity are unknown. We have identified an NKG2D-mediated signaling pathway that governs both conjugate formation and cytotoxic granule polarization. We demonstrate that an interaction between the regulatory subunit of PI3K, p85, and the adaptor protein CrkL is required for efficient NKG2D-mediated cellular cytotoxicity. We show decreased NK cell-target cell conjugate formation in NK cells treated with PI3K inhibitors or depleted of CrkL. Independent of adhesion, we find that microtubule organization center polarization toward target cells expressing the NKG2D ligand MICA or toward anti-NKG2D-coated beads is impaired in the absence of CrkL. Ab-stimulated granule release is also impaired in NK cells depleted of CrkL. Furthermore, our data indicate that the small Ras family GTPase Rap1 is activated downstream of NKG2D engagement in a PI3K- and CrkL-dependent manner and is required for conjugate formation, MTOC (microtubule organizing center) polarization, and NKG2D-dependent cellular cytotoxicity. Taken together, our data identify an NKG2D-activated signaling pathway that collectively orchestrates NK cell adhesion, cell polarization, and granule release.

ABSTRACTPneumocystisspecies are opportunistic fungal pathogens that cause severe pneumonia in immunocompromised hosts. Recent evidence has suggested that unidentified proteases are involved inPneumocystislife cycle regulation. Proteolytically active ADAM (named for “a disintegrin and metalloprotease”) family molecules have been identified in some fungal organisms, such asAspergillus fumigatusandSchizosaccharomyces pombe, and some have been shown to participate in life cycle regulation. Accordingly, we sought to characterize ADAM-like molecules in the fungal opportunistic pathogen,Pneumocystis carinii(PcADAM). After an in silico search of theP. cariniigenomic sequencing project identified a 329-bp partial sequence with homology to known ADAM proteins, the full-length PcADAMsequence was obtained by PCR extension cloning, yielding a final coding sequence of 1,650 bp. Sequence analysis detected the presence of a typical ADAM catalytic active site (HEXXHXXGXXHD). Expression of PcADAMover thePneumocystislife cycle was analyzed by Northern blot. Southern and contour-clamped homogenous electronic field blot analysis demonstrated its presence in theP. cariniigenome. Expression of PcADAMwas observed to be increased inPneumocystiscysts compared to trophic forms. The full-length gene was subsequently cloned and heterologously expressed inSaccharomyces cerevisiae. Purified PcADAMp protein was proteolytically active in casein zymography, requiring divalent zinc. Furthermore, native PcADAMp extracted directly from freshly isolatedPneumocystisorganisms also exhibited protease activity. This is the first report of protease activity attributable to a specific, characterized protein in the clinically important opportunistic fungal pathogenPneumocystis.