Institute of Biotechnology and Biomedicine (BIOTECMED)

Department of Microbiology and Ecology, and Institute of Biotechnology and Biomedicine (BIOTECMED)

University of Valencia

María Luisa Gil and Alberto Yáñez

(From left to right): Daniel Gozalbo, María Luisa Gil, Paula Guerrero,
Cristina Bono, Alberto Yáñez and Javier Megías.

The research group currently led by María Luisa Gil and Alberto Yáñez, called "Immunology of fungal infections", has focused its research, during the last fifteen years, on the study of the host's immune response against Candida albicans. The group has multidisciplinary training, both in the areas of Microbiology and Immunology, therefore it presents an ideal profile to study the interactions between pathogenic fungi and cells of the immune system both in vitro and in vivo. Although the research carried out is fundamentally basic, it has a clear potential applied in the development of new immunotherapeutic approaches for the treatment of fungal infections.

C. albicans is an opportunistic pathogen that, depending on the underlying host defect, is capable of causing a variety of infections ranging from superficial mucocutaneous candidiasis to severe invasive candidiasis. The frequency and severity of the latter has increased considerably in recent decades, due to the increase in the population at risk who are immunosuppressed or weakened by different causes. Resistance to candidiasis requires the coordinated action of innate and acquired immune defenses. The mature cells of the innate immune system use different PRRs (pattern recognition receptors) to directly recognize MAMPs (microorganism-associated molecular patterns), so that with a limited number of these receptors they can recognize a wide variety of pathogens. The most important families of PRRs in the recognition of C. albicans They are the Toll-like receptors (TLRs) and the C-type lectins (CLRs, such as dectin-1). In this context, our group demonstrated that the TLR2 receptor is involved in the recognition of C. albicans, both yeast and hyphae, inducing the secretion of cytokines and chemokines through a pathway dependent on the adapter molecule MyD88 and that such recognition is critical for protection against invasive candidiasis in a mouse model of infection.

In 2006 it was described that hematopoietic stem and progenitor cells (HSPCs), from which all cells of the immune system are derived, express functional TLRs, and that signaling via TLRs in hematopoietic stem cells (HSCs) causes their entry into cell cycle and its differentiation towards the myeloid lineage. This discovery opened new perspectives regarding host-pathogen interactions, since these receptors could participate in the modulation of hematopoiesis in response to microorganisms during an infection. At that time our group decided to study the participation of PRRs in the interaction of C. albicans with HSPCs and its consequences in the resolution of the infection. Working in this line we have shown that C. albicans induces the proliferation of HSPCs and their differentiation towards the myeloid lineage, both in vitro and in vivo. This response requires signaling via TLR2 and dectin-1, and gives rise to functional macrophages that are capable of internalizing and destroying yeast, in addition to secreting inflammatory cytokines. These results indicate that pathogens can be directly recognized by HSPCs through PRRs, thus promoting the replenishment capacity of the innate immune system during an infection. Therefore, these receptors could be, at least in part, responsible for the emergency myelopoiesis that occurs during most infections, including invasive candidiasis.

On the other hand, numerous recent studies have questioned the dogma that immunological memory is an exclusive characteristic of specific immunity, since cells of innate immunity can exhibit a certain "memory" and respond differently to a second encounter. with the same or another microbial stimulus. For example, exposure of monocytes and macrophages to C. albicans it increases its response to a second encounter (trained immunity, dependent on dectin-1), while ligands of TLR4 or TLR2 confer a lower inflammatory response to macrophages (tolerance). Parallel to studies on memory of innate immunity, our group set out as a new objective to study the function of phagocytes formed after contact of HSPCs with microbial ligands. Using in vitro and in vivo models, we have shown that stimulation of PRRs in HSPCs affects the functional phenotype of the macrophages they subsequently generate. Therefore, our results show that this new concept of "memory" of innate immunity can be applied, not only to mature myeloid cells, but also to HSPCs, which contributes to increasing the durability of innate memory over time. . Based on these results, and those of other authors in the same line, an active role is currently assigned to HSPCs in the fight against infection. The hypothesis we are currently working on is that HSPCs can directly detect microorganisms and contribute to protection against infection by different mechanisms, including their ability to differentiate into myeloid cells with an improved phenotype to deal with the pathogen and initiate infection. immune response. The results already obtained open new perspectives that may be of great interest in the intersection between Immunology, Microbiology and Hematology. The existence of new mechanisms in the host-pathogen interaction, and its consequences in the modulation of the immune response during infection, may represent a new target for intervention against serious infections by enhancing the immune response. In addition, the modulation of hematopoiesis by microorganisms could reveal new strategies for the treatment of diseases with alterations in the production of myeloid cells, such as myeloid leukemias.

In recent years, our group has been collaborating with other research groups that have common scientific interests, including collaboration with Helen Goodridge (Cedars-Sinai Medical Center, Los Angeles, CA, USA) and H. Leighton (Lee) Grimes (Cincinnati Children's Hospital Medical Center, Cincinnati, OH, USA).

CONTRIBUTION AND SELECTED PUBLICATIONS

1. Yáñez A, Murciano C, O'Connor JE, Gozalbo D and Gil ML. (2009). Candida albicans triggers proliferation and differentiation of hematopoietic stem and progenitor cells by a MyD88-dependent signaling. Microbes Infect 11: 531-535.
2. Yáñez A, Flores A, Murciano C, O’Connor JE, Gozalbo D and Gil ML. (2010). Signalling through TLR2/ MyD88 induces differentiation of murine bone marrow stem and progenitor cells to functional phagocytes in response to Candida albicans. Cell Microbiol 12: 114-128.
3. Yáñez A, Megías J, O'Connor JE, Gozalbo D and Gil ML. (2011). Candida albicans induces selective development of macrophages and monocyte derived dendritic cells by a TLR2 dependent signalling. PLoS One 6: e24761.
4. Megías J, Yáñez A, Moriano S, O'Connor JE, Gozalbo D and Gil ML. (2012). Direct Toll-like receptor-mediated stimulation of hematopoietic stem and progenitor cells occurs in vivo and promotes differentiation toward macrophages. Stem Cells 30: 1486-1495.
5. Megías J, Maneu V, Salvador P, Gozalbo D and Gil ML. (2012). Candida albicans stimulates in vivo differentiation of haematopoietic stem and progenitor cells towards macrophages by a TLR2-dependent signalling. Cell Microbiol 15: 1143-1153.
6. Yáñez A, Goodridge HS, Gozalbo D and Gil ML. (2013). TLRs control hematopoiesis during infection. Eur J Immunol 43: 2526-2533.
7. Yáñez A, Hassanzadeh-Kiabi N, Ng MY, Megías J, Subramanian A, Liu GY, Underhill DM, Gil ML and Goodridge HS. (2013). Detection of a TLR2 agonist by hematopoietic stem and progenitor cells impacts the function of the macrophages they produce. Eur J Immunol 43: 2114-2125.
8. Gil ML, Murciano C, Yáñez A and Gozalbo D. (2016). Role of Toll-like receptors in systemic Candida albicans infections. Front Biosci 21: 278-302.
9. Megías J, Martínez A, Yáñez A, Goodridge HS, Gozalbo D and Gil ML. (2016). TLR2, TLR4 and Dectin-1 signalling in hematopoietic stem and progenitor cells determines the antifungal phenotype of the macrophages they produce. Microbes Infect 18: 354-363.
10. Martínez A, Bono C, Megías J, Yáñez A, Gozalbo D and Gil ML. (2017). PRR signaling during in vitro macrophage differentiation from progenitors modulates their subsequent response to inflammatory stimuli. Eur Cytokine Netw 28:102-110.
11. Gozalbo D, Murciano C and Gil ML. (2017). Immune response to Candida albicans infection. In: Reference Module in Life Sciences. Elsevier, ISBN: 978-0-12- 809633-8, http:/dx.doi.org/10.1016/B978-0-12- 809633-8.12075-8.
12. Yáñez A, Coetzee SG, Olsson A, Muench DE, Berman BP, Hazelett DJ, Salomonis N, Grimes HL, Goodridge HS. (2017). Granulocyte-Monocyte Progenitors and Monocyte-Dendritic Cell Progenitors Independently Produce Functionally Distinct Monocytes. Immunity. 47: 890-902.e4.
13. Martínez A, Bono C, Megías J, Yáñez A, Gozalbo D and Gil ML. (2018). Systemic candidiasis and TLR2 agonist exposure impacts the antifungal response of hematopoietic stem and progenitor cells. Front Cell Infect Microbiol 8:309.
14. Yáñez A, Murciano C, Gil ML and Gozalbo D (2019). Immune response to Candida albicans infection. Encyclopedia of Mycology. Reference Module in Life Sciences. Chapter 10013 Elsevier.
15. Bono C, Martínez A, Megías J,, Gozalbo D, Yáñez A and Gil ML (2020). Dectin-1 stimulation of hematopoietic stem and progenitor cells occurs in vivo and promotes differentiation toward trained macrophages via an indirect cell-autonomous mechanism. mBio 23:11(3):e00781-20.
16. Martínez A, Bono C, Gozalbo D, Goodridge HS, Gil ML and Yáñez A. (2020). TLR2 and Dectin-1 signaling in mouse hematopoietic stem and progenitor cells impacts the ability of the antigen presenting cells they produce to activate CD4 T cells. Cells 9:1317.