Pro-inflammatory response ensured by LPS and Pam3CSK4 in RAW 264.7 cells did not improve a fungistatic effect on Cryptococcus gattii infection
Authors: de Campos Gabriela Yamazaki , Oliveira Raquel Amorim , Oliveira-Brito Patrícia Kellen Martins , Roque-Barreira Maria Cristina , da Silva Thiago Aparecido 
Published: November 2020

Background

The macrophage lineage is characterized by plasticity due to the acquisition of distinct functional phenotypes, and two major subsets are evaluated; classical M1 activation (strong microbicidal activity) and alternative M2 activation (immunoregulatory functions). The M1 subset expresses inducible nitric oxide synthase (iNOS), which is a primary marker to identify these cells, whereas M2 macrophages are characterized by expression of Arginase-1, found in inflammatory zone 1 (Fizz1), chitinase-like molecule (Ym-1), and CD206. The micro-environmental stimuli and signals in tissues are critical in the macrophage polarization. Toll-like receptors (TLR) ligands, such as lipopolysaccharide (LPS), palmitoyl-3-cysteine-serine-lysine-4 (Pam3CSK4), and ArtinM (mannose-binding lectin) are inductors of M1 subset. The impact of TLR2 and TLR4 signals to fight against Cryptococcus gattii infection is unknown, which is a fungal pathogen that preferentially infects the lung of immunocompetent individuals. The macrophages initiate an immune response to combat the C. gattii, then we evaluated in RAW 264.7 cell the effect of TLR2 and TLR4 agonists on the macrophage polarization dynamic and the impact on the growth of C. gattii.

Methods and Results

We demonstrated that P3C4, LPS, and ArtinM induced an increase in the levels of iNOS transcripts in RAW 264.7 cells, whereas the relative expression of arginase-1, Ym-1, and Fizz1 was significantly increased in the presence of IL-4 alone. The effects of TLR2 and TLR4 agonists on repolarization from the M2 to M1 subset was evaluated, and the first stimulus was composed of IL-4 and, after 24 h of incubation, the cells were submitted to a second stimulus of P3C4, LPS, ArtinM, or Medium. These TLR agonists induced the production of TNF-α in polarized RAW 264.7 cells to the M2 subset, moreover the measurement of M1/M2 markers using qRT-PCR demonstrated that a second stimulus with LPS for 24 h induced a significant augmentation of levels of iNOS mRNA. This impact of TLR2 and TLR4 agonists in the activation of the RAW 264.7 macrophage was assayed in the presence of C. gattii, the macrophages stimulated with TLR2 and TLR4 agonists for 24 h and co-cultured with C. gattii, as a second stimulus, reached high levels of TNF-α even after incubation with different concentrations of C. gattii. The activation of RAW 264.7 cells induced by TLR2 and TLR4 agonists favored the phagocytosis of C. gattii and inhibited the growth of yeast in the early period of infection. However, RAW 264.7 cells incubated with C. gattii in the presence of TLR2 and TLR4 agonists did not result a significant difference in the colony forming unit (CFU) assay in the early period of C. gattii infection, compared to negative control.

Conclusion

Polarized RAW 264.7 cells to the M1 subset with TLR2 and TLR4 agonists did not inhibit the growth of C. gattii, whereas robust immunity was identified that could dysregulate host tolerance to this pathogen.