The commensal fungus Candida albicans (Ca) is responsible for ~700,000 annual cases of systemic candidiasis globally, with an associated mortality rate of 40% despite concurrent antifungal therapy 1 . In cancer patients receiving hematopoietic cell transplant (HCT). systemic candidiasis is associated with significantly increased relapse-free mortality 2,3. A better understanding of the cellular and molecular anti-fungal immune responses holds a promise in improving outcomes. including for cancer patients undergoing IICT, as it may allow improved risk stratification for better prognosis and identification of novel molecules for antifungal immunotherapy. To this end the work proposed here aims to provide novel mechanistic insights into the roles of complement C3 and C5 signaling in local and systemic fungal control'. Complement C3 and C5 are essential for their roles in immunosurveillance and downstream immune effector function regulation. Upon pathogen encounter, rapid deposition, and cleavage of C3 into C3b/iC3b leads to opsonophagocytosis via CR1-4 4 • Additionally, C3b forms a multimeric convertase complex and cleaves C5 into the potent immune effector peptide C5a 5 • During the K99 phase of this project, I identified a crucial role for C5a in regulation of neutrophil- and macrophage-dependent antifungal effector functions to curb fungal overgrowth. immunometabolic dysregulation, and inflammatory renal damage to promote survival. Like C5a, the smaller C3 cleavage fragment- C3a is an equally important immunoregulatory peptide which signals via its receptor C3ar 4 ; however, its functions during systemic candidiasis remain completely unexplored. Furthermore, the cellular sources of C3/C5 in Candida-infected kidneys or Candida-colonized gastrointestinal tract post HCT, their regulation and their in1pacts on antifungal cellular network remain unexplored. I hypothesize that complement C3 and C5 play non-redundant roles in systemic and mucosa] fungal control. I will systematically assess this hypothesis during the R00 phase of this award via three specific aims. Under Aim1 I will characterize the cell-intrinsic mechanisms of C5 regulation; to this end. I will use C5 reporter mice. primary cells from mice/humans and a series of ex vivo stimulation and intracellular complement activation assays. Furthermore, I will assess the role of C5 in fungal colonization control during HCT using a mouse model ofHCT6 (Aim 1). I will also assess the role ofC3 in protective immunity during systemic candidiasis. where the use of C3-/-, C3ar-/- and cell-type specific C3/C3ar knockout mice will reveal the relative contributions of different bioactive fragments of C3 in protective antifungal response (Aim 2). Concurrently, I will begin delineating C3 and C5-dependent auto-/para-/endocrine cellular networks responsible for sterilizing renal antifungal immunity using in vivo antifungal effector function assays and cell-type specific knockout mice (Aim 3). Thus, support through this R00 award will be instrumental in completing this important research to fruition. which will provide strong foundation upon which further investigations will he pursued.