181. The Effects of in vitro LPS Stimulation on the PBMC Transcriptome of Beef Steers with Negative or Positive Residual Average Daily Gain.

Abstract This study characterized the peripheral blood mononuclear cell (PBMC) transcriptomic responses to in vitro lipopolysaccharide (LPS) stimulation in beef steers with divergent residual average daily gain (RADG), a residual-based index of feed efficiency. Positive-RADG (n = 6; RADG = +0.53 kg/d) and negative-RADG (n = 6; RADG = −0.46 kg/d) crossbred steers were selected from a cohort of 41 steers (average body weight = 384 ± 5.8 kg) following a 50-d performance testing period on a high-forage total mixed ration. PBMCs were isolated from positive-RADG and negative-RADG steers and assigned to LPS stimulation (20 μg/mL for 2 hours) or untreated control. RNA was extracted, sequenced and differentially expressed genes (DEGs; FDR ≤ 0.01, |log₂FC| ≥ 1) were identified by comparison with unstimulated controls. Gene ontology (GO) enrichment analysis was performed to characterize associated biological processes (P < 0.05). Prior to LPS stimulation, one DEG was identified between RADG groups, indicating near-equivalent baseline PBMC transcriptomes. Following stimulation, 2,739 DEGs were identified in positive-RADG steers (1,484 upregulated; 1,255 downregulated), and 3,299 DEGs in negative-RADG steers (1,555 upregulated; 1,744 downregulated). GO enrichment analysis revealed activation of canonical innate immune pathways in both groups, including cytokine production, positive regulation of IL-1 and IL-1β production, and immune response. However, positive-RADG steers exhibited a broader cytokine network enrichment profile, with the highest enrichment score for positive regulation of cytokine production, encompassing genes involved in effector activation, leukocyte trafficking, and inflammatory resolution, including NOS2, STAT3, PTX3, TGFB1, and SOD1. In contrast, negative-RADG steers exhibited disproportionate enrichment of IL-1β- and IL-6-centered inflammatory pathways, with SAA3 as the most highly upregulated DEG (log₂FC = 9.18 vs. 6.80 in positive-RADG steers), consistent with a more pronounced acute-phase transcriptional response. These findings demonstrate that while RADG-divergent steers share conserved LPS-responsive signaling pathways, they differ in the architecture of their immune transcriptional responses. The broader, more integrated cytokine network activation in positive-RADG steers, compared with the concentrated IL-1β/IL-6-driven inflammatory signature in negative-RADG steers, suggests that superior feed efficiency is associated with a more coordinated immunoregulatory capacity that may reduce the energetic cost of innate immune activation and facilitate return to metabolic homeostasis.