Intraperitoneal Injection of Bacterial Lipopolysaccharide in Mice to Study Systemic Immune Responses

Begin with an awake mouse, restrained to limit its movement.

Tilt the head downward to expose the lower abdomen.

Locate the abdominal midline and identify an injection site slightly off-center to avoid internal organs.

Next, take a syringe containing a solution of lipopolysaccharides, a component of Gram-negative bacterial cell envelope.

Insert the needle into the site to access the peritoneal cavity, and inject the solution.

Once injected, LPS enters systemic circulation and triggers an immune response.

In the bloodstream, LPS binds to LPS-binding protein, which transfers it to CD14, a membrane protein on immune cells.

CD14 delivers LPS to MD-2, a glycoprotein that associates with Toll-like receptor 4 (TLR4), enabling TLR4 dimerization.

Dimerized TLR4 recruits adaptor proteins that activate downstream signaling pathways, inducing proinflammatory cytokine expression and systemic inflammation.

This models microbial translocation and systemic immune activation in the intestines and other organs caused by bacterial products.

Handle the mouse gently but firmly, and restrain the animal in one hand. Ensure that the mouse is securely held and can breathe normally.

Tilt the mouse nose slightly towards the floor to expose the abdomen for injection. Locate the midline of the abdomen and inject the volume of LPS required on the lower-left or right side.

It is very important to ensure that the needle enters the peritoneal cavity to avoid misinjections.

The needle should also not go too deep into the peritoneal cavity to avoid injury of inner organs.

Gently return the animal to the home cage.

Monitor the mice for the occurrence and severity of endotoxemia at the time of injection and every 2 hours thereafter for 8 hours. Record observations in the attached score sheet.