CPO 4

Infectious-disease experiments test how pathogens interact with host barriers, innate immune receptors, inflammatory signaling, pathogen replication, and tissue injury; pattern-recognition receptors such as TLRs, RIG-I-like receptors, NOD-like receptors, and inflammasomes detect microbial molecules and activate NF-κB, interferon, and cytokine responses. The central hypothesis is that infection severity reflects the balance between pathogen burden and host response: protective inflammation restricts pathogen growth, whereas excessive or mislocalized inflammation contributes to tissue damage and disease phenotype. Unresolved questions include which host pathways are protective versus pathogenic, why some infection models fail to translate to human disease, and which combined readouts best predict clinically relevant infection outcomes.

PCNA immunodetection measures proliferative activity by detecting proliferating cell nuclear antigen, a nuclear protein associated with DNA polymerase δ function and DNA replication. The assay readout is the proportion of PCNA-positive nuclei among total counted cells, but PCNA labeling is not identical to BrdU labeling because PCNA can mark late G1/early S-associated replication competence and may persist beyond active DNA synthesis depending on fixation and extraction conditions.

The MTT assay is a colorimetric endpoint assay for estimating viable cell number, cell growth, cytotoxicity, or cell activation in cultured mammalian cells. Living cells reduce the yellow tetrazolium salt MTT into purple/blue formazan, while dead cells do not generate the same signal; the resulting color can be quantified with a multiwell spectrophotometer. MTT reduction is commonly interpreted as a readout of metabolic activity that often correlates with viable cell number, but it should not be treated as a direct cell-counting method unless the assay is optimized for the cell type and experimental condition. Studies show that MTT reduction can involve mitochondrial and non-mitochondrial reducing systems, and formazan may accumulate in intracellular lipid droplets rather than simply marking mitochondria.