Direct measurement of the force generated by a single macrophage

During immune responses, macrophages play important roles in phagocytosis and antigen presentation by engulfing pathogenic micro-organisms and cell debris. Since the function of a macrophage highly depends on a series of physical steps including migration, direct contact and strong binding to its target, deployment of cytoplasm and membrane, and intake of the target, here we have investigated the mechanical behavior of a macrophage by manipulating it with a flexible pipette that was used as a force sensor and transducer. We examined the response of a macrophage to mechanical pulling by a positively charged pipette. We observed that the macrophage initially formed strong binding to the pipette and migrated along the direction of pulling for some early period. After that period, the macrophage exerted a huge traction force to pull the pipette back and attempted to retract itself towards its original location. We found that whether it was able to return to the original location depended on the level of applied force. Since the traction force generated by a single macrophage had not been characterized accurately, we measured the force for the first time to our knowledge and found the maximum traction force to be around 80 nN. This quantitative measurement was made possible by a new and convenient method used to calibrate the stiffness of the pipette. Through the study, we acquired a better understanding of the mechanics of and the force generation by a macrophage.