Cytotoxic and Immunomodulatory Effects of Phormidesmis molle Extract on Human Cells In Vitro

Cyanobacteria of the genus <i>Phormidesmis</i> are recognized as a promising source of biologically active secondary metabolites with anticancer and immunomodulatory properties. In the present study, we investigated both the cytotoxic and immunological effects of an extract obtained from <i>Phormidesmis molle</i> PACC (Plovdiv Algal Culture Collection) 8140 as well as its chemical composition. The extract was profiled by LC-ESI-MS/MS (Liquid chromatography-electrospray ionization-tandem mass spectrometry), and selected compounds were evaluated with in silico ADMET (Absorption, distribution, metabolism, excretion and toxicity) modeling. The cytotoxic potential of the extract was evaluated in vitro using the MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide) assay on human colorectal adenocarcinoma cell lines (Caco-2, HT-29, and LS-180). The immunological impact of the extract was assessed on human peripheral blood mononuclear cells (PBMCs) isolated from healthy donors. PBMCs were treated with 100 µg/mL extract for 48 h, followed by flow cytometric immunophenotyping and ELISA (Enzyme-linked immunosorbent assay)-based cytokine quantification. The extract induced a concentration- and time-dependent decrease in cancer cell viability after 24, 48, and 72 h of exposure. At 72 h, treatment with the highest concentration (200 µg/mL) reduced cell viability to 74% in Caco-2 cells, 69-70% in HT-29 cells, and 59-61% in LS-180 cells. Morphological changes observed after treatment with <i>Phormidesmis</i> extract showed pronounced cytotoxic and apoptosis-related effects in the colorectal cancer cell lines tested. Immunophenotyping revealed a pronounced expansion of natural killer (NK) cells (CD56⁺ and/or CD16⁺). CD3^-CD56^-CD16⁺ NK population was markedly increased (from 67.7 ± 0.95% in non-treated PBMCs to 94.66 ± 0.90% in extract-treated PBMCs, <i>p</i> < 0.001). In contrast, the proportions of CD8⁺ T cells, CD19⁺ B cells, and CD11b⁺ monocytes were significantly reduced (from 21.5 ± 4.50% to 7.22 ± 0.41%, from 11.9 ± 1.70% to 6.06 ± 0.42%, and from 66.4 ± 0.60% to 34.4 ± 0.87%, respectively). Cytokine analysis demonstrated strong suppression of Th1-associated cytokines, with significantly reduced interferon gamma (IFN-γ, 461 ng/mL in controls vs. 84 ng/mL in extract-treated cultures) and tumor necrosis factor alpha (TNF-α) levels (169 ng/mL in controls vs. 32 ng/mL in extract-treated cultures), whereas nterleukin-6 (IL-6) was moderately elevated (from 158 ng/mL in controls to 234 ng/mL in extract-treated cultures) and IL-10 remained low. These findings demonstrate that <i>P. molle</i> extract combines cytotoxic activity against cancer cells with potent immunomodulatory effects, highlighting its potential as a source of bioactive compounds for immune-based therapeutic strategies.