Immunological correlates of protection mediated by a whole organism, Cryptococcus neoformans, vaccine deficient in chitosan.

The global burden of infections due to the pathogenic fungus Cryptococcus is substantial in persons with low CD4+ T-cell counts. Previously, we deleted three chitin deacetylase genes from Cryptococcus neoformans to create a chitosan-deficient, avirulent strain, designated as cda1∆2∆3∆, which, when used as a vaccine, protected mice from challenge with virulent C. neoformans strain KN99. Here, we explored the immunological basis for protection. Vaccine-mediated protection was maintained in mice lacking B cells or CD8+ T cells. In contrast, protection was lost in mice lacking α/ß T cells or CD4+ T cells. Moreover, CD4+ T cells from vaccinated mice conferred protection upon adoptive transfer to naive mice. Importantly, while monoclonal antibody-mediated depletion of CD4+ T cells just prior to vaccination resulted in complete loss of protection, significant protection was retained in mice depleted of CD4+ T cells after vaccination but prior to challenge. Vaccine-mediated protection was lost in mice genetically deficient in interferon-γ (IFNγ), tumor necrosis factor alpha (TNFα), or interleukin (IL)-23p19. A robust influx of leukocytes and IFNγ- and TNFα-expressing CD4+ T cells was seen in the lungs of vaccinated and challenged mice. Finally, a higher level of IFNγ production by lung cells stimulated ex vivo correlated with lower fungal burden in the lungs. Thus, while B cells and CD8+ T cells are dispensable, IFNγ and CD4+ T cells have overlapping roles in generating protective immunity prior to cda1∆2∆3∆ vaccination. However, once vaccinated, protection becomes less dependent on CD4+ T cells, suggesting a strategy for vaccinating HIV+ persons prior to loss of CD4+ T cells. IMPORTANCE: The fungus Cryptococcus neoformans is responsible for >100,000 deaths annually, mostly in persons with impaired CD4+ T-cell function such as AIDS. There are no approved human vaccines. We previously created a genetically engineered avirulent strain of C. neoformans, designated as cda1∆2∆3∆. When used as a vaccine, cda1∆2∆3∆ protects mice against a subsequent challenge with a virulent C. neoformans strain. Here, we defined components of the immune system responsible for vaccine-mediated protection. We found that while B cells and CD8+ T cells were dispensible, protection was lost in mice genetically deficient in CD4+ T cells and the cytokines IFNγ, TNFα, or IL-23. A robust influx of cytokine-producing CD4+ T cells was seen in the lungs of vaccinated mice following infection. Importantly, protection was retained in mice depleted of CD4+ T cells following vaccination, suggesting a strategy to protect persons who are at risk of future CD4+ T-cell dysfunction.

Immunological correlates of protection mediated by a whole organism Cryptococcus neoformans vaccine deficient in chitosan.

The global burden of infections due to the pathogenic fungus Cryptococcus is substantial in persons with low CD4 + T cell counts. Previously, we deleted three chitin deacetylase genes from C. neoformans to create a chitosan-deficient, avirulent strain, designated cda1Δ2Δ3Δ which, when used as a vaccine, protected mice from challenge with virulent C. neoformans strain KN99. Here, we explored the immunological basis for protection. Vaccine-mediated protection was maintained in mice lacking B cells or CD8 + T cells. In contrast, protection was lost in mice lacking α/ß T cells or CD4 + T cells. Moreover, CD4 + T cells from vaccinated mice conferred protection upon adoptive transfer to naive mice. Importantly, while monoclonal antibody-mediated depletion of CD4 + T cells just prior to vaccination resulted in complete loss of protection, significant protection was retained in mice depleted of CD4 + T cells after vaccination, but prior to challenge. Vaccine-mediated protection was lost in mice genetically deficient in IFNγ, TNFα, or IL-23p19. A robust influx of leukocytes and IFNγ- and TNFα-expressing CD4 + T cells was seen in the lungs of vaccinated and challenged mice. Finally, a higher level of IFNγ production by lung cells stimulated ex vivo correlated with lower fungal burden in the lungs. Thus, while B cells and CD8 + T cells are dispensable, IFNγ and CD4 + T cells have overlapping roles in generating protective immunity prior to cda1Δ2Δ3Δ vaccination. However, once vaccinated, protection becomes less dependent on CD4 + T cells, suggesting a strategy for vaccinating HIV + persons prior to loss of CD4 + T cells. Importance: The fungus Cryptococcus neoformans is responsible for >100,000 deaths annually, mostly in persons with impaired CD4 + T cell function such as AIDS. There are no approved human vaccines. We previously created a genetically engineered avirulent strain of C. neoformans , designated cda1Δ2Δ3Δ . When used as a vaccine, cda1Δ2Δ3Δ protects mice against a subsequent challenge with a virulent C. neoformans strain. Here, we defined components of the immune system responsible for vaccine-mediated protection. We found that while B cells and CD8 + T cells were dispensible, protection was lost in mice genetically deficient in CD4 + T cells, and the cytokines IFNγ, TNFα, or IL-23. A robust influx of cytokine-producing CD4 + T cells was seen in the lungs of vaccinated mice following infection. Importantly, protection was retained in mice depleted of CD4 + T cells following vaccination, suggesting a strategy to protect persons who are at risk for future CD4 + T cell dysfunction.

Fluorescence and Biochemical Assessment of the Chitin and Chitosan Content of Cryptococcus.

The cell wall of the fungal pathogens Cryptococcus neoformans and C. gattii is critical for cell wall integrity and signaling external threats to the cell, allowing it to adapt and grow in a variety of changing environments. Chitin is a polysaccharide found in the cell walls of fungi that is considered to be essential for fungal survival. Chitosan is a polysaccharide derived from chitin via deacetylation that is also essential for cryptococcal cell wall integrity, fungal pathogenicity, and virulence. Cryptococcus has evolved mechanisms to regulate the amount of chitin and chitosan during growth under laboratory conditions or during mammalian infection. Therefore, levels of chitin and chitosan have been useful phenotypes to define mutant Cryptococcus strains. As a result, we have developed and/or refined various qualitative and quantitative methods for measuring chitin and chitosan. These techniques include those that use fluorescent probes that are known to bind to chitin (e.g., calcofluor white and wheat germ agglutinin), as well as those that preferentially bind to chitosan (e.g., eosin Y and cibacron brilliant red 3B-A). Techniques that enhance the localization and quantification of chitin and chitosan in the cell wall include (i) fluorescence microscopy, (ii) flow cytometry, (iii) and spectrofluorometry. We have also modified two highly selective biochemical methods to measure cellular chitin and chitosan content: the Morgan-Elson and the 3-methyl-2-benzothiazolone hydrazine hydrochloride (MBTH) assays, respectively.

Chitosan-Deficient Cryptococcus as Whole-Cell Vaccines.

Creating a safe and effective vaccine against infection by the fungal pathogen Cryptococcus neoformans is an appealing option that complements the discovery of new small molecule antifungals. Recent animal studies have yielded promising results for a variety of vaccines that include live-attenuated and heat-killed whole-cell vaccines, as well as subunit vaccines formulated around recombinant proteins. Some of the recombinantly engineered cryptococcal mutants in the chitosan biosynthesis pathway are avirulent and very effective at conferring protective immunity. Mice vaccinated with these avirulent chitosan-deficient strains are protected from a lethal pulmonary infection with C. neoformans strain KN99. Heat-killed derivatives of the vaccination strains are likewise effective in a murine model of infection. The efficacy of these whole-cell vaccines, however, is dependent on a number of factors, including the inoculation dose, route of vaccination, frequency of vaccination, and the specific mouse strain used in the study. Here, we present detailed methods for identifying and optimizing various factors influencing vaccine potency and efficacy in various inbred mouse strains using a chitosan-deficient cda1Δcda2Δcda3Δ strain as a whole-cell vaccine candidate. This chapter describes the protocols for immunizing three different laboratory mouse strains with vaccination regimens that use intranasal, orotracheal, and subcutaneous vaccination routes after the animals were sedated using two different types of anesthesia.

Cell wall composition in Cryptococcus neoformans is media dependent and alters host response, inducing protective immunity.

Introduction: Cryptococcus neoformans is a basidiomycete fungus that can cause meningoencephalitis, especially in immunocompromised patients. Cryptococcus grows in many different media, although little attention has been paid to the role of growth conditions on the cryptococcal cell wall or on virulence. Objective: The purpose of this study was to determine how different media influenced the amount of chitin and chitosan in the cell wall, which in turn impacted the cell wall architecture and host response. Methods: Yeast extract, peptone, and dextrose (YPD) and yeast nitrogen base (YNB) are two commonly used media for growing Cryptococcus before use in in vitro or in vivo experiments. As a result, C. neoformans was grown in either YPD or YNB, which were either left unbuffered or buffered to pH 7 with MOPS. These cells were then labeled with cell wall-specific fluorescent probes to determine the amounts of various cell wall components. In addition, these cells were employed in animal virulence studies using the murine inhalation model of infection. Results: We observed that the growth of wild-type C. neoformans KN99 significantly changes the pH of unbuffered media during growth. It raises the pH to 8.0 when grown in unbuffered YPD but lowers the pH to 2.0 when grown in unbuffered YNB (YNB-U). Importantly, the composition of the cell wall was substantially impacted by growth in different media. Cells grown in YNB-U exhibited a 90% reduction in chitosan, the deacetylated form of chitin, compared with cells grown in YPD. The decrease in pH and chitosan in the YNB-U-grown cells was associated with a significant increase in some pathogen-associated molecular patterns on the surface of cells compared with cells grown in YPD or YNB, pH 7. This altered cell wall architecture resulted in a significant reduction in virulence when tested using a murine model of infection. Furthermore, when heat-killed cells were used as the inoculum, KN99 cells grown in YNB-U caused an aberrant hyper-inflammatory response in the lungs, resulting in rapid animal death. In contrast, heat-killed KN99 cells grown in YNB, pH 7, caused little to no inflammatory response in the host lung, but, when used as a vaccine, they conferred a robust protective response against a subsequent challenge infection with the virulent KN99 cells. Conclusion: These findings emphasize the importance of culture media and pH during growth in shaping the content and organization of the C. neoformans cell wall, as well as their impact on fungal virulence and the host response.

Cryptococcus neoformans Cda1 and Cda2 coordinate deacetylation of chitin during infection to control fungal virulence.

Chitosan, a deacetylated form of chitin, is required for the virulence of Cryptococcus neoformans. There are three chitin deacetylase genes (CDA) that are essential for chitosan production, and deletion of all three genes results in the absence of chitosan, loss of virulence, and induction of a protective host response when used as a vaccine. Cda1 plays a major role in deacetylating chitin during pulmonary infection of CBA/J mice. Inoculation with the cda1Δ strain did not lead to a lethal infection. However, the infection was not cleared. The persistence of the fungus in the host suggests that chitin is still being deacetylated by Cda2 and/or Cda3. To test this hypothesis, we subjected strains deleted of two CDA genes to fungal virulence in CBA/J, C57BL/6 and BALB/c and found that cda1Δcda2Δ was avirulent in all mouse lines, as evidenced by its complete clearance. Consistent with the major role of Cda1 in CBA/J, we found that cda2Δcda3Δ was as virulent as its wild-type progenitor KN99. On the other hand, cda1Δcda3Δ displayed virulence comparable to that of cda1Δ. The virulence of each mutant correlates with the amount of chitosan produced when grown under host-mimicking culture conditions. In addition, the avirulence of cda1Δcda2Δ was followed by the induction of a protective immune response in C57BL/6 and CBA/J mice, when a live or heat-killed form of the mutant was used as a vaccine respectively. Taken together, these data imply that, in C. neoformans, coordinated activity of both Cda1 and Cda2 is essential for mediating fungal virulence.

Cryptococcus neoformans Chitin Synthase 3 Plays a Critical Role in Dampening Host Inflammatory Responses.

Cryptococcus neoformans infections are significant causes of morbidity and mortality among AIDS patients and the third most common invasive fungal infection in organ transplant recipients. One of the main interfaces between the fungus and the host is the fungal cell wall. The cryptococcal cell wall is unusual among human-pathogenic fungi in that the chitin is predominantly deacetylated to chitosan. Chitosan-deficient strains of C. neoformans were found to be avirulent and rapidly cleared from the murine lung. Moreover, infection with a chitosan-deficient C. neoformans strain lacking three chitin deacetylases (cda1Δcda2Δcda3Δ) was found to confer protective immunity to a subsequent challenge with a virulent wild-type counterpart. In addition to the chitin deacetylases, it was previously shown that chitin synthase 3 (Chs3) is also essential for chitin deacetylase-mediated formation of chitosan. Mice inoculated with the chs3Δ strain at a dose previously shown to induce protection with the cda1Δcda2Δcda3Δ strain die within 36 h after installation of the organism. Mortality was not dependent on viable fungi, as mice inoculated with a heat-killed preparation of the chs3Δ strain died at the same rate as mice inoculated with a live chs3Δ strain, suggesting that the rapid onset of death was host mediated, likely caused by an overexuberant immune response. Histology, cytokine profiling, and flow cytometry indicate a massive neutrophil influx in the mice inoculated with the chs3Δ strain. Mice depleted of neutrophils survived chs3Δ inoculation, indicating that death was neutrophil mediated. Altogether, these studies lead us to conclude that Chs3, along with chitosan, plays critical roles in dampening cryptococcus-induced host inflammatory responses.IMPORTANCECryptococcus neoformans is the most common disseminated fungal pathogen in AIDS patients, resulting in ∼200,000 deaths each year. There is a pressing need for new treatments for this infection, as current antifungal therapy is hampered by toxicity and/or the inability of the host's immune system to aid in resolution of the disease. An ideal target for new therapies is the fungal cell wall. The cryptococcal cell wall is different from the cell walls of many other pathogenic fungi in that it contains chitosan. Strains that have decreased chitosan are less pathogenic and strains that are deficient in chitosan are avirulent and can induce protective responses. In this study, we investigated the host responses to a chs3Δ strain, a chitosan-deficient strain, and found that mice inoculated with the chs3Δ strain all died within 36 h and that death was associated with an aberrant hyperinflammatory immune response driven by neutrophils, indicating that chitosan is critical in modulating the immune response to Cryptococcus.

The Aminoalkylindole BML-190 Negatively Regulates Chitosan Synthesis via the Cyclic AMP/Protein Kinase A1 Pathway in Cryptococcus neoformans.

Cryptococcus neoformans can cause fatal meningoencephalitis in patients with AIDS or other immunocompromising conditions. Current antifungals are suboptimal to treat this disease; therefore, novel targets and new therapies are needed. Previously, we have shown that chitosan is a critical component of the cryptococcal cell wall and is required for survival in the mammalian host and that chitosan deficiency results in rapid clearance from the mammalian host. We had also identified several specific proteins that were required for chitosan biosynthesis, and we hypothesize that screening for compounds that inhibit chitosan biosynthesis would identify additional genes/proteins that influence chitosan biosynthesis. To identify these compounds, we developed a robust and novel cell-based flow cytometry screening method to identify small-molecule inhibitors of chitosan production. We screened the ICCB Known Bioactives library and identified 8 compounds that reduced chitosan in C. neoformans We used flow cytometry-based counterscreens and confirmatory screens, followed by a biochemical secondary screen to refine our primary screening hits to 2 confirmed hits. One of the confirmed hits that reduced chitosan content was the aminoalkylindole BML-190, a known inverse agonist of mammalian cannabinoid receptors. We demonstrated that BML-190 likely targets the C. neoformans G-protein-coupled receptor Gpr4 and, via the cyclic AMP (cAMP)/protein kinase A (PKA) signaling pathway, contributes to an intracellular accumulation of cAMP that results in decreased chitosan. Our discovery suggests that this approach could be used to identify additional compounds and pathways that reduce chitosan biosynthesis and could lead to potential novel therapeutics against C. neoformansIMPORTANCECryptococcus neoformans is a fungal pathogen that kills ∼200,000 people every year. The cell wall is an essential organelle that protects fungi from the environment. Chitosan, the deacetylated form of chitin, has been shown to be an essential component of the cryptococcal cell wall during infection of a mammalian host. In this study, we screened a set of 480 compounds, which are known to have defined biological activities, for activity that reduced chitosan production in C. neoformans Two of these compounds were confirmed using an alternative method of measuring chitosan, and one of these was demonstrated to impact the cAMP signal transduction pathway. This work demonstrates that the cAMP pathway regulates chitosan biosynthesis in C. neoformans and validates that this screening approach could be used to find potential antifungal agents.

Chitosan Biosynthesis and Virulence in the Human Fungal Pathogen Cryptococcus gattii.

Cryptococcus gattii R265 is a hypervirulent fungal strain responsible for the recent outbreak of cryptococcosis in Vancouver Island of British Columbia in Canada. It differs significantly from Cryptococcus neoformans in its natural environment, its preferred site in the mammalian host, and its pathogenesis. Our previous studies of C. neoformans have shown that the presence of chitosan, the deacetylated form of chitin, in the cell wall attenuates inflammatory responses in the host, while its absence induces robust immune responses, which in turn facilitate clearance of the fungus and induces a protective response. The results of the present investigation reveal that the cell wall of C. gattii R265 contains a two- to threefold larger amount of chitosan than that of C. neoformans The genes responsible for the biosynthesis of chitosan are highly conserved in the R265 genome; the roles of the three chitin deacetylases (CDAs) have, however, been modified. To deduce their roles, single and double CDA deletion strains and a triple CDA deletion strain were constructed in a R265 background and were subjected to mammalian infection studies. Unlike C. neoformans where Cda1 has a discernible role in fungal pathogenesis, in strain R265, Cda3 is critical for virulence. Deletion of either CDA3 alone or in combination with another CDA (cda1Δ3Δ or cda2Δ3Δ) or both (cda1Δ2Δ3Δ) rendered the fungus avirulent and cleared from the infected host. Moreover, the cda1Δ2Δ3Δ strain of R265 induced a protective response to a subsequent infection with R265. These studies begin to illuminate the regulation of chitosan biosynthesis of C. gattii and its subsequent effect on fungal virulence.IMPORTANCE The fungal cell wall is an essential organelle whose components provide the first line of defense against host-induced antifungal activity. Chitosan is one of the carbohydrate polymers in the cell wall that significantly affects the outcome of host-pathogen interaction. Chitosan-deficient strains are avirulent, implicating chitosan as a critical virulence factor. C. gattii R265 is an important fungal pathogen of concern due to its ability to cause infections in individuals with no apparent immune dysfunction and an increasing geographical distribution. Characterization of the fungal cell wall and understanding the contribution of individual molecules of the cell wall matrix to fungal pathogenesis offer new therapeutic avenues for intervention. In this report, we show that the C. gattii R265 strain has evolved alternate regulation of chitosan biosynthesis under both laboratory growth conditions and during mammalian infection compared to that of C. neoformans.

Cryptococcus neoformans Cda1 and Its Chitin Deacetylase Activity Are Required for Fungal Pathogenesis.

Chitin is an essential component of the cell wall of Cryptococcus neoformans conferring structural rigidity and integrity under diverse environmental conditions. Chitin deacetylase genes encode the enyzmes (chitin deacetylases [Cdas]) that deacetylate chitin, converting it to chitosan. The functional role of chitosan in the fungal cell wall is not well defined, but it is an important virulence determinant of C. neoformans Mutant strains deficient in chitosan are completely avirulent in a mouse pulmonary infection model. C. neoformans carries genes that encode three Cdas (Cda1, Cda2, and Cda3) that appear to be functionally redundant in cells grown under vegetative conditions. Here we report that C. neoformans Cda1 is the principal Cda responsible for fungal pathogenesis. Point mutations were introduced in the active site of Cda1 to generate strains in which the enzyme activity of Cda1 was abolished without perturbing either its stability or localization. When used to infect CBA/J mice, Cda1 mutant strains produced less chitosan and were attenuated for virulence. We further demonstrate that C. neoformans Cda genes are transcribed differently during a murine infection from what has been measured in vitroIMPORTANCECryptococcus neoformans is unique among fungal pathogens that cause disease in a mammalian host, as it secretes a polysaccharide capsule that hinders recognition by the host to facilitate its survival and proliferation. Even though it causes serious infections in immunocompromised hosts, reports of infection in hosts that are immunocompetent are on the rise. The cell wall of a fungal pathogen, its synthesis, composition, and pathways of remodelling are attractive therapeutic targets for the development of fungicides. Chitosan, a polysaccharide in the cell wall of C. neoformans is one such target, as it is critical for pathogenesis and absent in the host. The results we present shed light on the importance of one of the chitin deacetylases that synthesize chitosan during infection and further implicates chitosan as being a critical factor for the pathogenesis of C. neoformans.

A fluorogenic C. neoformans reporter strain with a robust expression of m-cherry expressed from a safe haven site in the genome.

C. neoformans is an encapsulated fungal pathogen with defined asexual and sexual life cycles. Due to the availability of genetic and molecular tools for its manipulation, it has become a model organism for studies of fungal pathogens, even though it lacks a reliable system for maintaining DNA fragments as extrachromosomal plasmids. To compensate for this deficiency, we identified a genomic gene-free intergenic region where heterologous DNA could be inserted by homologous recombination without adverse effects on the phenotype of the recipient strain. Since such a site in the C. neoformans genome at a different location has been named previously as "safe haven", we named this locus second safe haven site (SH2). Insertion of DNA into this site in the genome of the KN99 congenic strain pair caused minimal change in the growth of the engineered strain under a variety of in vitro and in vivo conditions. We exploited this 'safe' locus to create a genetically stable highly fluorescent strain expressing mCherry protein (KN99mCH); this strain closely resembled its wild-type parent (KN99α) in growth under a variety of in vitro stress conditions and in the expression of virulence traits. The efficiency of phagocytosis and the proliferation of KN99mCH inside human monocyte-derived macrophages were comparable to those of KN99α, and the engineered strain showed the expected organ dissemination after inoculation, although there was a slight reduction in virulence. The mCherry fluorescence allowed us to measure specific association of cryptococci with leukocytes in the lungs and mediastinal lymph nodes of infected animals and, for the first-time, to assess their live/dead status in vivo. These results highlight the utility of KN99mCH for elucidation of host-pathogen interactions in vivo. Finally, we generated drug-resistant KN99 strains of both mating types that are marked at the SH2 locus with a specific drug resistant gene cassette; these strains will facilitate the generation of mutant strains by mating.

Induction of Protective Immunity to Cryptococcal Infection in Mice by a Heat-Killed, Chitosan-Deficient Strain of Cryptococcus neoformans.

UNLABELLED: Cryptococcus neoformans is a major opportunistic fungal pathogen that causes fatal meningoencephalitis in immunocompromised individuals and is responsible for a large proportion of AIDS-related deaths. The fungal cell wall is an essential organelle which undergoes constant modification during various stages of growth and is critical for fungal pathogenesis. One critical component of the fungal cell wall is chitin, which in C. neoformans is predominantly deacetylated to chitosan. We previously reported that three chitin deacetylase (CDA) genes have to be deleted to generate a chitosan-deficient C. neoformans strain. This cda1Δ2Δ3Δ strain was avirulent in mice, as it was rapidly cleared from the lungs of infected mice. Here, we report that clearance of the cda1Δ2Δ3Δ strain was associated with sharply spiked concentrations of proinflammatory molecules that are known to be critical mediators of the orchestration of a protective Th1-type adaptive immune response. This was followed by the selective enrichment of the Th1-type T cell population in the cda1Δ2Δ3Δ strain-infected mouse lung. Importantly, this response resulted in the development of robust protective immunity to a subsequent lethal challenge with a virulent wild-type C. neoformans strain. Moreover, protective immunity was also induced in mice vaccinated with heat-killed cda1Δ2Δ3Δ cells and was effective in multiple mouse strains. The results presented here provide a strong framework to develop the cda1Δ2Δ3Δ strain as a potential vaccine candidate for C. neoformans infection. IMPORTANCE: The most commonly used anticryptococcal therapies include amphotericin B, 5-fluorocytosine, and fluconazole alone or in combination. Major drawbacks of these treatment options are their limited efficacy, poor availability in limited resource areas, and potential toxicity. The development of antifungal vaccines and immune-based therapeutic interventions is promising and an attractive alternative to chemotherapeutics. Currently, there are no fungal vaccines in clinical use. This is the first report of a C. neoformans deletion strain with an avirulent phenotype in mice exhibiting protective immunity when used as a vaccine after heat inactivation, although other strains that overexpress fungal or murine proteins have recently been shown to induce a protective response. The data presented here demonstrate the potential for developing the avirulent cda1Δ2Δ3Δ strain into a vaccine-based therapy to treat C. neoformans infection.

Role of Cryptococcus neoformans Rho1 GTPases in the PKC1 signaling pathway in response to thermal stress.

To initiate and establish infection in mammals, the opportunistic fungal pathogen Cryptococcus neoformans must survive and thrive upon subjection to host temperature. Primary maintenance of cell integrity is controlled through the protein kinase C1 (PKC1) signaling pathway, which is regulated by a Rho1 GTPase in Saccharomyces cerevisiae. We identified three C. neoformans Rho GTPases, Rho1, Rho10, and Rho11, and have begun to elucidate their role in growth and activation of the PKC1 pathway in response to thermal stress. Western blot analysis revealed that heat shock of wild-type cells resulted in phosphorylation of Mpk1 mitogen-activated protein kinase (MAPK). Constitutive activation of Rho1 caused phosphorylation of Mpk1 independent of temperature, indicating its role in pathway regulation. A strain with a deletion of RHO10 also displayed this constitutive Mpk1 phosphorylation phenotype, while one with a deletion of RHO11 yielded phosphorylation similar to that of wild type. Surprisingly, like a rho10Δ strain, a strain with a deletion of both RHO10 and RHO11 displayed temperature sensitivity but mimicked wild-type phosphorylation, which suggests that Rho10 and Rho11 have coordinately regulated functions. Heat shock-induced Mpk1 phosphorylation also required the PKC1 pathway kinases Bck1 and Mkk2. However, Pkc1, thought to be the major regulatory kinase of the cell integrity pathway, was dispensable for this response. Together, our results argue that Rho proteins likely interact via downstream components of the PKC1 pathway or by alternative pathways to activate the cell integrity pathway in C. neoformans.