Abstract
Throughout evolution, cytomegaloviruses (CMVs) have been capturing genes from their hosts, employing the derived proteins to evade host immune defenses. We have recently reported the presence of a number of CD48 homologs (vCD48s) encoded by different pathogenic viruses, including several CMVs. However, their properties and biological relevance remain as yet unexplored. CD48, a cosignaling molecule expressed on the surface of most hematopoietic cells, modulates the function of natural killer (NK) and other cytotoxic cells by binding to its natural ligand 2B4 (CD244). Here, we have characterized A43, the vCD48 exhibiting the highest amino acid sequence identity with host CD48. A43, which is encoded by owl monkey CMV, is a soluble molecule released from the cell after being proteolytically processed through its membrane proximal region. A43 is expressed with immediate-early kinetics, yielding a protein that is rapidly detected in the supernatant of infected cells. Remarkably, surface plasmon resonance assays revealed that this viral protein binds to host 2B4 with high affinity and slow dissociation rates. We demonstrate that soluble A43 is capable to abrogate host CD48:2B4 interactions. Moreover, A43 strongly binds to human 2B4 and prevents 2B4-mediated NK-cell adhesion to target cells, therefore reducing the formation of conjugates and the establishment of immunological synapses between human NK cells and CD48-expressing target cells. Furthermore, in the presence of this viral protein, 2B4-mediated cytotoxicity and IFN-γ production by NK cells are severely impaired. In summary, we propose that A43 may serve as a functional soluble CD48 decoy receptor by binding and masking 2B4, thereby impeding effective NK cell immune control during viral infections. Thus, our findings provide a novel example of the immune evasion strategies developed by viruses.
Highlights
Natural killer (NK) cells are circulating lymphocytes that play a pivotal role in the rapid recognition and control of viral infections
We show that A43, a CD48 homolog encoded by owl monkey CMV, is a soluble molecule that exhibits exceptional binding kinetics for 2B4, and is capable of blocking the interaction with its counter-receptor CD48
We reveal how this viral protein interferes with human NK cell-mediated cytotoxicity by inhibiting the immune synapse between human NK cells and target cells
Summary
Natural killer (NK) cells are circulating lymphocytes that play a pivotal role in the rapid recognition and control of viral infections. The net balance of activating/inhibitory signals determines whether the NK cell will initiate its cytolytic activity through the degranulation of specialized secretory lysosomes into the immune synapse, causing the destruction of the target cell. One such receptor is 2B4 (or CD244), a member of the signaling lymphocyte activation molecule (SLAM) family of the immunoglobulin (Ig) superfamily [2]. 2B4 interacts with CD48, another member of the SLAM family that is broadly expressed on the surface of most hematopoietic cells [4,5,6] Both receptors contain an ectodomain composed of an N-terminal Ig membrane-distal variable (IgV) domain followed by an Ig constant-2-set domain, characterized by conserved cysteines. The 2B4:CD48 interaction contributes to the regulation of additional aspects of the innate and adaptive immune responses
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