The 'Fault' Lies in the Choroid: Peripapillary Intrachoroidal Cavitation Presenting with Progressive Vision Loss.

A 55-year-old male was referred to the Neuro-ophthalmology clinic due to gradual onset, progressive vision loss. On fundus examination a subtle yellow-orange peripapillary lesion was detected in the left eye. Optical coherence tomography with radial scanning illustrated retinal nerve fibre layer thinning as well as an area of intrachoroidal cavitation that corresponded to the lesion. Visual field testing showed a left inferior arcuate defect. Magnetic resonance imaging of the brain and orbit, and laboratory testing was unremarkable. Clinical examination, imaging, and testing were consistent with peripapillary intrachoroidal cavitation (PICC). Follow-up with serial visual field testing showed mild progression of the field defect. While PICC is not well understood in the literature, studies have reported associated risk factors including pathological myopia, older age, increased ocular axial length, chorioretinal atrophy, and vascular abnormalities. Importantly, glaucoma-like visual field defects as well as structural changes have been noticed in a high proportion of patients with PICC. While these alterations are evident, the pathogenic relationship between them is yet to be uncovered. Treatment with anti-glaucoma medications has been suggested, however, the evidence remains scarce for its true benefits. Care providers must be aware of the presentation of a yellow-orange peripapillary lesion with an associated visual field defect to accurately diagnose and manage this condition.

CD19-targeted chimeric antigen receptor T-cell therapy for CNS relapsed or refractory acute lymphocytic leukaemia: a post-hoc analysis of pooled data from five clinical trials.

BACKGROUND: CNS relapse of acute lymphocytic leukaemia is difficult to treat. Durable remissions of relapsed or refractory B-cell acute lymphocytic leukaemia have been observed following treatment with CD19-directed chimeric antigen receptor (CAR) T cells; however, most trials have excluded patients with active CNS disease. We aimed to assess the safety and activity of CAR T-cell therapy in patients with a history of CNS relapsed or refractory B-cell acute lymphocytic leukaemia. METHODS: In this post-hoc analysis, we included 195 patients (aged 1-29 years; 110 [56%] male and 85 [44%] female) with relapsed or refractory CD19-positive acute lymphocytic leukaemia or lymphocytic lymphoma from five clinical trials (Pedi CART19, 13BT022, ENSIGN, ELIANA, and 16CT022) done at the Children's Hospital of Philadelphia (Philadelphia, PA, USA), in which participants received CD19-directed CAR T-cell therapy between April 17, 2012, and April 16, 2019. The trials required control of CNS disease at enrolment and infusion and excluded treatment in the setting of acute neurological toxic effects (>grade 1 in severity) or parenchymal lesions deemed to increase the risk of neurotoxicity. 154 patients from Pedi CART19, ELIANA, ENSIGN, and 16CT022 received tisagenlecleucel and 41 patients from the 13BT022 trial received the humanised CD19-directed CAR, huCART19. We categorised patients into two strata on the basis of CNS status at relapse or within the 12 months preceding CAR T-cell infusion-either CNS-positive or CNS-negative disease. Patients with CNS-positive disease were further divided on the basis of morphological bone marrow involvement-either combined bone marrow and CNS involvement, or isolated CNS involvement. Endpoints were the proportion of patients with complete response at 28 days after infusion, Kaplan-Meier analysis of relapse-free survival and overall survival, and the incidence of cytokine release syndrome and neurotoxicity. FINDINGS: Of all 195 patients, 66 (34%) were categorised as having CNS-positive disease and 129 (66%) as having CNS-negative disease, and 43 (22%) were categorised as having isolated CNS involvement. The median length of follow-up was 39 months (IQR 25-49) in the CNS-positive stratum and 36 months (18-49) in the CNS-negative stratum. The proportion of patients in the CNS-positive stratum with a complete response at 28 days after infusion was similar to that in the CNS-negative stratum (64 [97%] of 66 vs 121 [94%] of 129; p=0·74), with no significant difference in relapse-free survival (60% [95% CI 49-74] vs 60% [51-71]; p=0·50) or overall survival (83% [75-93] vs 71% [64-79]; p=0·39) at 2 years between the two groups. Overall survival at 2 years was significantly higher in patients with isolated CNS involvement compared with those with bone marrow involvement (91% [82-100] vs 71% [64-78]; p=0·046). The incidence and severity of neurotoxicity (any grade, 53 [41%] vs 38 [58%]; grade 1, 24 [19%] vs 20 [30%]; grade 2, 14 [11%] vs 10 [15%]; grade 3, 12 [9%] vs 6 [9%], and grade 4, 3 [2%] vs 2 [3%]; p=0·20) and cytokine release syndrome (any grade, 110 [85%] vs 53 [80%]; grade 1, 12 [9%] vs 2 [3%]; grade 2, 61 [47%] vs 38 [58%]; grade 3, 18 [14%] vs 7 [11%] and grade 4, 19 [15%] vs 6 [9%]; p=0·26) did not differ between the CNS-negative and the CNS-positive disease strata. INTERPRETATION: Tisagenlecleucel and huCART19 are active at clearing CNS disease and maintaining durable remissions in children and young adults with CNS relapsed or refractory B-cell acute lymphocytic leukaemia or lymphocytic lymphoma, without increasing the risk of severe neurotoxicity; although care should be taken in the timing of therapy and disease control to mitigate this risk. These preliminary findings support the use of these CAR T-cell therapies for patients with CNS relapsed or refractory B-cell acute lymphocytic leukaemia. FUNDING: Children's Hospital of Philadelphia Frontier Program.

Risk-Adapted Preemptive Tocilizumab to Prevent Severe Cytokine Release Syndrome After CTL019 for Pediatric B-Cell Acute Lymphoblastic Leukemia: A Prospective Clinical Trial.

PURPOSE: To prospectively evaluate the effectiveness of risk-adapted preemptive tocilizumab (PT) administration in preventing severe cytokine release syndrome (CRS) after CTL019, a CD19 chimeric antigen receptor T-cell therapy. METHODS: Children and young adults with CD19-positive relapsed or refractory B-cell acute lymphoblastic leukemia were assigned to high- (≥ 40%) or low- (< 40%) tumor burden cohorts (HTBC or LTBC) based on a bone marrow aspirate or biopsy before infusion. HTBC patients received a single dose of tocilizumab (8-12 mg/kg) after development of high, persistent fevers. LTBC patients received standard CRS management. The primary end point was the frequency of grade 4 CRS (Penn scale), with an observed rate of ≤ 5 of 15 patients in the HTBC pre-defined as clinically meaningful. In post hoc analyses, the HTBC was compared with a historical cohort of high-tumor burden patients from the initial phase I CTL019 trial. RESULTS: The primary end point was met. Seventy patients were infused with CTL019, 15 in the HTBC and 55 in the LTBC. All HTBC patients received the PT intervention. The incidence of grade 4 CRS was 27% (95% CI, 8 to 55) in the HTBC and 3.6% (95% CI, 0.4 to 13) in the LTBC. The best overall response rate was 87% in the HTBC and 100% in the LTBC. Initial CTL019 expansion was greater in the HTBC than the LTBC (P < .001), but persistence was not different (P = .73). Event-free and overall survival were worse in the HTBC (P = .004, P < .001, respectively). In the post hoc analysis, grade 4 CRS was observed in 27% versus 50% of patients in the PT and prior phase I cohorts, respectively (P = .18). CONCLUSION: Risk-adapted PT administration resulted in a decrease in the expected incidence of grade 4 CRS, meeting the study end point, without adversely impacting the antitumor efficacy or safety of CTL019.

Chimeric antigen receptor T cell therapy for pediatric and young adult B cell acute lymphoblastic leukemia.

INTRODUCTION: Though 85% of children and young adults with acute lymphoblastic leukemia (ALL) are cured, until recently, the prognosis of relapsed or refractory disease has been dismal. The advent of chimeric antigen receptor (CAR) T-cell therapy has transformed the treatment of relapsed/refractory ALL. The most well-studied, successful CARs are autologous, murine-based anti-CD19 CARs, but new constructs are currently under clinical investigation. AREAS COVERED: This review describes the history and design of CAR T cells, clinical trial outcomes of anti-CD19 and newer CARs, treatment-related toxicities including cytokine release syndrome and neurotoxicity, and issues with resistance and relapse. A search of PubMed and clinicaltrials.gov spanning from 2012-present was used to select original reports investigating the use of CAR T in pediatric patients. EXPERT OPINION: CD19-targeted CARs have demonstrated remarkable response rates and produced durable remissions in very high-risk pediatric patient populations. The therapies, however, are limited by unique treatment-related toxicities and considerable rates of antigen-positive and antigen-negative relapses. Current research efforts focused on elucidating mechanisms of resistance/relapse and on developing strategies to prevent and treat relapse are critical to optimizing the use of CAR-T. In addition, ongoing trials testing CARs earlier in therapy and for new indications are key to informing their widespread usage.

Gain-of-Function STAT1 Mutation With Familial Lymphadenopathy and Hodgkin Lymphoma.

In this report, we describe a novel T437N STAT1 mutation found in a mother and 3 of her 4 children which we demonstrate yields gain-of-function. All of the four patients with the T437N STAT1 mutation experienced lymphadenopathy. However, two of the children developed Nodular Lymphocyte Predominant Hodgkin Lymphoma (NHLPL) and have responded to chemotherapeutic regimens. The fourth sibling had neither the STAT1 variant nor lymphadenopathy or malignancy. To our knowledge this is the first description of a potential association between STAT1 GOF mutations and lymphoma development.

A life-threatening flecainide overdose treated with intravenous fat emulsion.

Flecainide is a Vaughan Williams Class Ic antidysrhythmic associated with PR, QRS, and QTc prolongation on the electrocardiogram and development of life-threatening cardiac toxicity in overdose. The cornerstone of treatment is fluid resuscitation and the administration of magnesium and sodium bicarbonate. We report a case of flecainide overdose associated with life-threatening hemodynamic compromise successfully treated with intravenous fat emulsion (IFE) therapy. IFE should be considered as a novel adjunctive therapy in patients with life-threatening toxicity following flecainide overdose.

Homeostatic tissue responses in skin biopsies from NOMID patients with constitutive overproduction of IL-1ß.

The autoinflammatory disorder, Neonatal-onset Multisystem Inflammatory Disease (NOMID) is the most severe phenotype of disorders caused by mutations in CIAS1 that result in increased production and secretion of active IL-1ß. NOMID patients present with systemic and organ-specific inflammation of the skin, central nervous system and bone, and respond dramatically to treatment with IL-1 blocking agents. We compared the cellular infiltrates and transcriptome of skin biopsies from patients with NOMID (n = 14) before treatment (lesional (LS) and non-lesional (pre-NL) skin) and after treatment (post-NL) with the IL-1 blocker anakinra (recombinant IL-1 receptor antagonist, Kineret®, Swedish Orphan Biovitrum AB, SOBI), to normal skin (n = 5) to assess tissue responses in the context of untreated and treated disease. Abundant neutrophils distinguish LS skin from pre-NL and post-NL skin. CD11c(+) dermal dendritic cells and CD163(+) macrophages expressed activated caspase-1 and are a likely source of cutaneous IL-1 production. Treatment with anakinra led to the disappearance of neutrophils, but CD3(+) T cells and HLA-DR(+) cells remained elevated. Among the upregulated genes IL-6, IL-8, TNF, IL-17A, CCL20, and the neutrophil defensins DEFA1 and DEFA3 were differentially regulated in LS tissues (compared to normal skin). Important significantly downregulated pathways in LS skin included IL-1R/TLR signaling, type I and II cytokine receptor signaling, mitochondrial dysfunction, and antigen presentation. The differential expression and regulation of microRNAs and pathways involved in post-transcriptional modification were suggestive of epigenetic modification in the chronically inflamed tissue. Overall, the dysregulated genes and pathways suggest extensive "adaptive" mechanisms to control inflammation and maintain tissue homeostasis, likely triggered by chronic IL-1 release in the skin of patients with NOMID.

Identification of FoxM1/Bub1b signaling pathway as a required component for growth and survival of rhabdomyosarcoma.

We identified Bub1b as an essential element for the growth and survival of rhabdomyosarcoma (RMS) cells using a bar-coded, tetracycline-inducible short hairpin RNA (shRNA) library screen. Knockdown of Bub1b resulted in suppression of tumor growth in vivo, including the regression of established tumors. The mechanism by which this occurs is via postmitotic endoreduplication checkpoint and mitotic catastrophe. Furthermore, using a chromatin immunoprecipitation assay, we found that Bub1b is a direct transcriptional target of Forkhead Box M1 (FoxM1). Suppression of FoxM1 either by shRNA or the inhibitor siomycin A resulted in reduction of Bub1b expression and inhibition of cell growth and survival. These results show the important role of the Bub1b/FoxM1 pathway in RMS and provide potential therapeutic targets.

Sustained response and prevention of damage progression in patients with neonatal-onset multisystem inflammatory disease treated with anakinra: a cohort study to determine three- and five-year outcomes.

OBJECTIVE: Blocking interleukin-1 with anakinra in patients with the autoinflammatory syndrome neonatal-onset multisystem inflammatory disease (NOMID) reduces systemic and organ-specific inflammation. However, the impact of long-term treatment has not been established. This study was undertaken to evaluate the long-term effect of anakinra on clinical and laboratory outcomes and safety in patients with NOMID. METHODS: We conducted a cohort study of 26 NOMID patients ages 0.80-42.17 years who were followed up at the NIH and treated with anakinra 1-5 mg/kg/day for at least 36 months. Disease activity was assessed using daily diaries, questionnaires, and C-reactive protein level. Central nervous system (CNS) inflammation, hearing, vision, and safety were evaluated. RESULTS: Sustained improvements in diary scores, parent's/patient's and physician's global scores of disease activity, parent's/patient's pain scores, and inflammatory markers were observed (all P<0.001 at 36 and 60 months). At 36 and 60 months, CNS inflammation was suppressed, with decreased cerebrospinal fluid white blood cell counts (P=0.0026 and P=0.0076, respectively), albumin levels, and opening pressures (P=0.0012 and P<0.001, respectively). Most patients showed stable or improved hearing. Cochlear enhancement on magnetic resonance imaging correlated with continued hearing loss. Visual acuity and peripheral vision were stable. Low optic nerve size correlated with poor visual field. Bony lesions progressed. Adverse events other than viral infections were rare, and all patients continued to receive the medication. CONCLUSION: These findings indicate that anakinra provides sustained efficacy in the treatment of NOMID for up to 5 years, with the requirement of dose escalation. Damage progression in the CNS, ear, and eye, but not bone, is preventable. Anakinra is well tolerated overall.

An oleate-stimulated, phosphatidylinositol 4,5-bisphosphate-independent phospholipase D in Schizosaccharomyces pombe.

Phospholipase D1 (PLD1) is an important enzyme involved in lipid-mediated signal transduction and membrane dynamics in eukaryotes. PLD1 preferentially hydrolyzes phosphatidylcholine to phosphatidic acid. This potent second messenger is involved in cytoskeletal reorganization, secretion, and membrane trafficking in eukaryotic cells. In Saccharomyces cerevisiae, PLD1 is involved in polarized growth and morphogenesis during pheromone response and sporulation. The presence of a PLD activity in Schizosaccharomyces pombe is demonstrated. PLD activity was able to hydrolyze a fluorescently labeled analog of phosphatidylcholine and was capable of performing the transphosphatidylation reaction characteristic of PLDs. Schizosaccharomyces pombe PLD activity was unaffected by phosphatidylinositol 4,5 bisphosphate (PIP(2)), but was slightly stimulated by oleate. PLD activity was shown to increase when the S. pombe cells underwent mating and sporulation. Here, we also report the molecular cloning of the first phospholipase D isoform from an S. pombe genomic DNA library (EMBL accession no. FN547388). Comparisons of three divergent yeasts, S. pombe, S. cerevisiae, and Candida albicans, with respect to the PLD enzymes revealed differences in regulation by oleate and PIP(2). Even with high homology in the protein sequences between the PLD1 enzymes of S. cerevisiae, C. albicans, and S. pombe, there was variation with the effects of the regulators.

An upstream regulator and downstream target of phospholipase D1 activity during pheromone response in Saccharomyces cerevisiae.

Phospholipase D1 (PLD1), which is the product of the SPO14 gene, has been shown to play a role in the process of polarized cell growth (PCG) during the pheromone response in Saccharomyces cerevisiae. PLD1 hydrolyzes phosphatidylcholine to produce phosphatidic acid (PA) and a free choline headgroup. This study investigated the interactions of PLD1 and PA with two proteins known to be involved in the cellular signaling leading to PCG in yeast, the small GTPase Cdc42p and the PAK family kinase Ste20p. Constitutively activated Cdc42p stimulates PLD1 activity. Protein-lipid binding blots confirmed the specific binding of Ste20p to the PLD1 product, PA. Finally, kinase activity assays provided evidence for the stimulation of Ste20p by PA. These findings highlight the important interactions among PLD1, Cdc42p and Ste20p during PCG in S. cerevisiae.

Candida albicans PLD I activity is required for full virulence.

Phospholipase D1 (PLD1) mutants of Candida albicans are defective in important in vivo and in vitro virulence factors. PLD1 mutants colonize the murine alimentary tract as well as PLD1 sufficient strains. In comparison to PLD1 sufficient strains, the PLD1 mutants: (i) are unable to survive in internal organs after intravenous challenge; (ii) do not decrease the body weights of mice after oral challenge; and (iii) are not lethal for immunodeficient mice after oral challenge. In vitro, the PLD1 mutants show a drastically reduced capacity to penetrate epithelial monolayers and they fail to develop hyphae when grown on solid Spider medium. The morphogenic switch from yeast to hyphae is controlled by multiple parallel signaling pathways that couple specific stimuli to the regulation of several transcription factors. Our data suggest that PLD1 functions in at least one of these pathways regulating morphogenesis in vitro and that while the mutants are able to form hyphae in vivo, the hyphae are defective in their ability to cause oroesophageal and gastric candidiasis and to kill the C. albicans-colonized mice.

Use of genetically engineered phage to deliver antimicrobial agents to bacteria: an alternative therapy for treatment of bacterial infections.

The emergence and increasing prevalence of multidrug-resistant bacterial pathogens emphasizes the need for new and innovative antimicrobial strategies. Lytic phages, which kill their host following amplification and release of progeny phage into the environment, may offer an alternative strategy for combating bacterial infections. In this study, however, we describe the use of a nonlytic phage to specifically target and deliver DNA encoding bactericidal proteins to bacteria. To test the concept of using phage as a lethal-agent delivery vehicle, we used the M13 phagemid system and the addiction toxins Gef and ChpBK. Phage delivery of lethal-agent phagemids reduced target bacterial numbers by several orders of magnitude in vitro and in a bacteremic mouse model of infection. Given the powerful genetic engineering tools available and the present knowledge in phage biology, this technology may have potential use in antimicrobial therapies and DNA vaccine development.

Doc-mediated cell killing in Shigella flexneri using a C1/LacI controlled expression system.

In this report we describe the development of a highly stringent and dually regulated promoter system for Shigella flexneri. Dual regulation was provided by utilizing a promoter susceptible to control by the bacteriophage P1 temperature-sensitive C1 repressor that in turn was under the transcriptional control of LacI. The level of induction/repression ratios observed was up to 3700-fold in S. flexneri. The general utility of this promoter system was evaluated by demonstrating that the bacteriophage P1 post-segregational killer protein Doc mediates a bactericidal effect in S. flexneri. This represents the first report of Doc (death on curing)-mediated killing in this Gram-negative species.

Propranolol inhibits hyphal development in Candida albicans.

Propranolol was used to investigate the role of phosphatidic acid (PA) and diacylglycerol in the dimorphic transition in Candida albicans. Propranolol was able to inhibit the appearance of germ tubes without decreasing growth rate. Data suggest that inhibition of morphogenesis may be due to binding by propranolol of PA derived from PLD1 hydrolysis of phosphatidylcholine.

Phosphatidylinositol-4-phosphate 5-kinase activity is stimulated during temperature-induced morphogenesis in Candida albicans.

Phosphoinositides are important lipid signalling molecules in eukaryotic cells. Phosphatidylinositol-4-phosphate 5-kinase (PI4P5K) catalyses the production of phosphatidylinositol 4,5-bisphosphate (PIP2), which stimulates phospholipase D1 (PLD1) activity in mammalian and yeast cells. PLD1 catalyses the formation of phosphatidic acid (PA), which has been shown to activate PI4P5Ks in mammalian and Saccharomyces cerevisiae cells. In the present study, PI4P5K activity in the opportunistic pathogen Candida albicans was identified. A gene with significant sequence homology to the S. cerevisiae PI4P5K was cloned and designated MSS4. This gene was demonstrated to encode a functional PI4P5K by expression in S. cerevisiae. This enzyme was found to be membrane-associated and was stimulated by PA. Within the first 20 min after induction of polarized hyphal growth induced by a shift to elevated temperature, PI4P5K activity increased 2.5-fold. This stimulation was not observed when hyphae were induced by a combination of elevated temperature and serum. A lack of PLD1 activity resulted in the loss of induction of PI4P5K activity during the morphogenetic switch. Furthermore, the addition of propranolol attenuated the stimulation of PI4P5K activity during morphogenesis. These results suggest that PA derived from PLD1 activity stimulates C. albicans PI4P5K during the switch to the hyphal form under some conditions.

Tight regulation and modulation via a C1-regulated promoter in Escherichia coli and Pseudomonas aeruginosa.

We describe the development and analysis of a novel promoter system regulated by the bacteriophage P1 temperature-sensitive C1 repressor. Using transcriptional fusions to the lacZ reporter gene to monitor gene expression, we show that the ratio of induction/repression can be up to 1500-fold in Escherichia coli. The promoters exhibited extremely tight repression and could be modulated over a range of temperatures. The utility of the promoter system was tested in Pseudomonas aeruginosa. C1 effectively repressed transcription; however, only modest induction was achieved. To increase the levels of induction, the amount of c1 was modulated at the mRNA level by using a LacI-regulated promoter. This resulted in a 59-fold induction in gene expression under inducing conditions. As the promoter system was constructed in a broad-host range vector and utilized the C1 repressor from a broad-host range phage, the system will provide the potential for controlled gene expression in Gram-negative bacteria.

Overcoming the phage replication threshold: a mathematical model with implications for phage therapy.

Prior observations of phage-host systems in vitro have led to the conclusion that susceptible host cell populations must reach a critical density before phage replication can occur. Such a replication threshold density would have broad implications for the therapeutic use of phage. In this report, we demonstrate experimentally that no such replication threshold exists and explain the previous data used to support the existence of the threshold in terms of a classical model of the kinetics of colloidal particle interactions in solution. This result leads us to conclude that the frequently used measure of multiplicity of infection (MOI), computed as the ratio of the number of phage to the number of cells, is generally inappropriate for situations in which cell concentrations are less than 10(7)/ml. In its place, we propose an alternative measure, MOI(actual), that takes into account the cell concentration and adsorption time. Properties of this function are elucidated that explain the demonstrated usefulness of MOI at high cell densities, as well as some unexpected consequences at low concentrations. In addition, the concept of MOI(actual) allows us to write simple formulas for computing practical quantities, such as the number of phage sufficient to infect 99.99% of host cells at arbitrary concentrations.

Development of a P1 phagemid system for the delivery of DNA into Gram-negative bacteria.

The inability to transform many clinically important Gram-negative bacteria has hampered genetic studies addressing the mechanism of bacterial pathogenesis. This report describes the development and construction of a delivery system utilizing the broad-host-range transducing bacteriophage P1. The phagemids used in this system contain a P1 pac initiation site to package the vector, a P1 lytic replicon to generate concatemeric DNA, a broad-host-range origin of replication and an antibiotic-resistance determinant to select bacterial clones containing the recircularized phagemid. Phagemid DNA was successfully introduced by infection and stably maintained in members of the families Enterobacteriaceae (Escherichia coli, Shigella flexneri, Shigella dysenteriae, Klebsiella pneumoniae and Citrobacter freundii) and Pseudomonadaceae (Pseudomonas aeruginosa). In addition to laboratory strains, these virions were used successfully to deliver phagemids to a number of strains isolated from patients. This ability to deliver genetic information to wild-type strains raises the potential for use in antimicrobial therapies and DNA vaccine development.

The role and relevance of phospholipase D1 during growth and dimorphism of Candida albicans.

The phosphatidylcholine-specific phospholipase D1 (PLD1) in Saccharomyces cerevisiae is involved in vesicle transport and is essential for sporulation. The gene encoding the homologous phospholipase D1 from Candida albicans (PLD1) was used to study the role of PLD1 in this pathogenic fungus. In vitro and in vivo expression studies using Northern blots and reverse transcriptase-PCR showed low PLD1 mRNA levels in defined media supporting yeast growth and during experimental infection, while enhanced levels of PLD1 transcripts were detected during the yeast to hyphal transition. To study the relevance of PLD1 during yeast and hyphal growth, an essential part of the gene was deleted in both alleles of two isogenic strains. In vitro PLD1 activity assays showed that pld1 mutants produced no detectable levels of phosphatidic acid, the hydrolytic product of PLD1 activity, and strongly reduced levels of diacylglycerol, the product of lipid phosphate phosphohydrolase, suggesting no or a negligible background PLD1 activity in the pld1 mutants. The pld1 mutants showed no growth differences compared to the parental wild-type in liquid complex and minimal media, independent of the growth temperature. In addition, growth rates of pld1 mutants in media with protein as the sole source of nitrogen were similar to growth rates of the wild-type, indicating that secretion of proteinases was not reduced. Chlamydospore formation was normal in pld1 mutants. When germ tube formation was induced in liquid media, pld1 mutants showed similar rates of yeast to hyphal transition compared to the wild-type. However, no hyphae formation was observed on solid Spider medium, and cell growth on cornmeal/Tween 80 medium indicated aberrant morphogenesis. In addition, pld1 mutants growing on solid media had an attenuated ability to invade the agar. In a model of oral candidosis, pld1 mutants showed no attenuation of virulence. In contrast, the mutant was less virulent in two different mouse models. These data suggest that PLD1 is not essential for growth and oral infections. However, they also suggest that a prominent part of the phosphatidic acid and diacylglycerol pools is produced by PLD1 and that the level of these components is important for morphological transitions under certain conditions in C. albicans.