TLR4 and pulmonary arterial hypertension: The role of the other monocyte subpopulation, CD14-positive monocytes, has been addressed by Trombetta et al. who showed an association between CD14 positive mixed M1/M2 cell subsets (CD14+CD206+CD163+CD204+TLR4+CD80+CD86+) and development of SSc-PAH [31].