Cold exposure induces vaso-occlusion and pain in sickle mice that depend on complement activation

Vaso-occlusive pain episodes (VOE) cause severe pain in sickle cell disease (SCD) patients. Vaso-occlusive events promote ischemia/reperfusion (I/R) pathobiology that activates complement. We hypothesized that complement activation is linked to VOE. We used cold to induce VOE in the Townes sickle (HbSS) mouse model and complement inhibitors to determine whether anaphylatoxin C5a mediates VOE. We used a dorsal skinfold chamber to measure microvascular stasis (vaso-occlusion) and von Frey filaments applied to the plantar surface of the hind paw to assess mechanical hyperalgesia in HbSS and control (HbAA) mice after cold-exposure (10oC/50oF) for 1 hour. Cold-exposure induced more vaso-occlusion in non-hyperalgesic HbSS mice (33%) than in HbAA mice (11%) or HbSS mice left at room temperature (1%). Cold-exposure also produced mechanical hyperalgesia as measured by paw withdrawal threshold in HbSS mice as compared to HbAA mice or HbSS mice left at room temperature. Vaso-occlusion and hyperalgesia were associated with an increase in complement activation fragments Bb and C5a in plasma of HbSS mice after cold exposure. This was accompanied by an increase in proinflammatory NF-κB activation and VCAM-1 and ICAM-1 adhesion molecule expression in the liver. Pretreatment of non-hyperalgesic HbSS mice before cold-exposure with anti-C5 or anti-C5aR monoclonal antibody (mAb) decreased vaso-occlusion, mechanical hyperalgesia, complement activation, and liver inflammatory markers as compared to pretreatment with control mAb. Anti-C5 or C5aR mAb infusion also abrogated mechanical hyperalgesia in HbSS mice with on-going hyperalgesia at baseline. These findings suggest that C5a promotes vaso-occlusion, pain, and inflammation during VOE and may play a role in chronic pain.