Reduced Virulence of HWP1-Deficient Mutants of Candida albicans and Their Interactions with Host Cells

The Candida albicans gene HWP1 encodes a surface protein that is required for normal hyphal development in vitro. We used mutants lacking one or both alleles of HWP1 to investigate the role of this gene in virulence. Mice infected intravenously with the homozygous hwp1 null mutant, CAL3, survived a median of >14 days, whereas mice infected with a control strain containing two functional alleles of HWP1 survived only 3.5 days. After 1 day of infection, all strains produced similar levels of infection in the kidneys, spleen, and blood. However, after 2 and 3 days, there was a significant decrease in the number of organisms in the kidneys of the mice infected with CAL3. This finding suggests that the hwp1 homozygous null mutant is normal in its ability to initiate infection but deficient in its capacity to maintain infection. CAL3 also germinated minimally in the kidneys. The ability of the heterozygous null mutant to germinate and cause mortality in mice was intermediate to CAL3, suggesting a gene dosage effect. To investigate potential mechanisms for the diminished virulence of CAL3, we examined its interactions with endothelial cells and neutrophils in vitro. CAL3 caused less endothelial cell injury than the heterozygous hwp1 mutant. We conclude that the HWP1 gene product is important for both in vivo hyphal development and pathogenicity of C. albicans. Also, the ability to form filaments may be critical for candidal virulence by enabling the fungus to induce cellular injury and maintain a deep-seated infection.

白念珠菌（Candida albicans）基因HWP1编码一种表面蛋白，该蛋白是体外（in vitro）正常菌丝发育所必需的。本研究利用缺失HWP1单等位或双等位基因的突变体，探究该基因在念珠菌毒力中的作用。经静脉感染纯合hwp1缺失突变株CAL3的小鼠，中位存活时长>14天；而感染携带两个功能性HWP1等位基因的对照菌株的小鼠，中位存活时长仅为3.5天。感染1天后，所有菌株在小鼠肾脏、脾脏与血液中的载菌量均无显著差异。但在感染第2天和第3天时，感染CAL3的小鼠肾脏中的载菌量出现显著下降。该结果表明，hwp1纯合缺失突变体启动感染的能力正常，但维持感染的能力存在缺陷。CAL3在小鼠肾脏中的菌丝萌发程度同样极低。杂合缺失突变体的菌丝萌发能力与小鼠致死率均介于CAL3与对照菌株之间，提示存在基因剂量效应。为探究CAL3毒力减弱的潜在机制，本研究在体外检测了其与内皮细胞及中性粒细胞的相互作用。相较于hwp1杂合缺失突变体，CAL3引发的内皮细胞损伤程度更低。本研究得出结论：HWP1基因编码的蛋白对白念珠菌的体内（in vivo）菌丝发育与致病性均具有重要作用。此外，菌丝形成能力可能是念珠菌毒力的关键决定因素——该能力可帮助真菌诱导细胞损伤并维持深部感染。