QPX (Quahog parasite unknown) is a protistan parasite affecting the hard clam Mercenaria mercenaria along the Northeastern coast of the United States. The geographic distribution and occurrence of disease epizootics suggests the involvement of environmental parameters and host genotype in disease development. Field investigations and laboratory transmission studies revealed some variations in the susceptibility of different hard clam stocks to QPX infection. Histological observations demonstrated that clams mount marked inflammatory responses against the infection that can sometimes lead to the healing of infected individuals. This study investigated host-pathogen interactions in QPX disease by studying individual components of the tripartite interaction: The pathogen, the host and the environment. Methods were developed to investigate virulence factors of the parasite QPX. Constitutive defense factors and immune response to QPX challenge in hard clams exhibiting different susceptibility to the parasite were also assessed using cellular, biochemical and molecular approaches. Finally, integrative approaches were used to determine the effect of environmental parameters (temperature, salinity, dissolved oxygen) on constitutive and QPX-induced clam defenses as well as on QPX itself. Results demonstrated that QPX produces virulence factors that are cytotoxic to M. mercenaria hemocytes. QPX cytotoxicity appears to be induced by clam factors and QPX extracellular products modulated host response against the parasite. In vitro growth of QPX was significantly reduced at temperatures above and below optimal levels ranging from 20 øC to 23 øC. QPX appeared to tolerate a wide range of salinities (22 to 37 psu) and parasite growth was only significantly affected at very low salinity (15 psu). Different QPX isolates displayed various levels of cytotoxicity against clam hemocytes and had different temperature optima supporting previous studies suggesting the existence of different QPX strains. This study also demonstrated the presence of constitutive compounds in clam tissues that significantly inhibited (mantle, gills) or stimulated (adductor muscle) QPX growth in vitro. Difference in anti-QPX activity was also observed in plasma from different clam populations and was associated to their respective susceptibility to QPX. Molecular investigations of clam response to QPX led to the identification of several stress- and defense-related genes such as ferritin, metallothionein, defensins, lectins and a thioester-containing protein (or TEP). Components of signaling pathways (such as NF-kB) were also characterized and their transcriptional regulation was correlated to the abundance of some humoral transcripts. Further investigation of the modulation of hemolymph parameters and transcriptional regulation of ferritin and metallothionein during bacterial and QPX challenge indicated difference in clam response according to the challenge. Similarly, comparison of host response among QPX-resistant and. susceptible clam broodstocks exhibited significant difference. Effective response of resistant clams against QPX led to the elimination of the parasite and the restoration of constitutive defense status whereas susceptible clams exhibited a strong modulation characterized by an acute phase response but appeared less effective to eliminate QPX after 28 days. Finally, results demonstrated significant effect of temperature and salinity on clam constitutive defenses and also on host. pathogen interaction and QPX disease dynamics. For instance, low temperature (13 øC) impeded clam defenses causing QPX disease development whereas changes in hemolymph parameters among QPX-infected clams maintained at 21 øC were related to significant healing processes. Higher QPX-associated mortality was also observed in infected clams maintained at high salinity (30 psu) compared to 15 psu. Overall, this study provided essential insights on QPX virulence, M. mercenaria defenses and effects of environmental parameters on host. pathogen interactions and provided perspectives for the remediation of QPX disease. The results not only improved our knowledge of cellular and molecular pathways involved during QPX disease, but also generated important information on M. mercenaria defenses, and enhanced our understanding of invertebrate immunity in general. Generated molecular information significantly enhanced public databases and allowed the development of new tools for the investigation of M. mercenaria transcriptome.