The role of CD16(-) and CD16(+) Mo subsets in human TB remains unknown. Our aim was to characterize Mo subsets from TB patients and to assess whether the inflammatory milieu from TB pleurisy modulate their phenotype and recruitment. We found an expansion of peripheral CD16(+) Mo that correlated with disease severity and with TNF-α plasma levels. Circulating Mo from TB patients are activated, showing a higher CD14, CD16, and CD11b expression and Mtb binding than HS. Both subsets coexpressed CCR2/CCR5, showing a potential ability to migrate to the inflammatory site. In tuberculous PF, the CD16(+) subset was the main Mo/MΦ population, accumulation that can be favored by the induction of CD16 expression in CD16(-) Mo triggered by soluble factors found in this inflammatory milieu. CD16(+) Mo in PF were characterized by a high density of receptors for Mtb recognition (DC-SIGN, MR, CD11b) and for lipid-antigens presentation (CD1b), allowing them to induce a successful, specific T cell proliferation response. Hence, in tuberculous PF, CD16(+) Mo constitute the main APC population; whereas in PB, their predominance is associated with the severity of pulmonary TB, suggesting a paradoxical role of the CD16(+) Mo subset that depends on the cellular localization.

Tuberculosis (TB) is one of the world's most pernicious diseases mainly due to immune evasion strategies displayed by its causative agent Mycobacterium tuberculosis (Mtb). Blood monocytes (Mos) represent an important source of DCs during chronic infections; consequently, the alteration of their differentiation constitutes an escape mechanism leading to mycobacterial persistence. We evaluated whether the CD16 + /CD16 − Mo ratio could be associated with the impaired Mo differentiation into DCs found in TB patients. The phenotype and ability to stimulate Mtb-specific memory clones DCs from isolated Mo subsets were assessed. We found that CD16 − Mos differentiated into CD1a + DC-SIGN high cells achieving an efficient recall response, while CD16 + Mos differentiated into a CD1a − DC-SIGN low population characterized by a poor mycobacterial Ag-presenting capacity. The high and sustained phosphorylated p38 expression observed in CD16 + Mos was involved in the altered DC profile given that its blockage restored DC phenotype and its activation impaired CD16 − Mo differentiation. Furthermore, depletion of CD16 + Mos indeed improved the differentiation of Mos from TB patients toward CD1a + DC-SIGN high DCs. Therefore, Mos from TB patients are less prone to differentiate into DCs due to their increased proportion of CD16 + Mos, suggesting that during Mtb infection Mo subsets may have different fates after entering the lungs. Keywords: Dendritic cells r Monocyte differentiation r Tuberculosis See accompanying Commentary by Lugo-Villarino and NeyrollesAdditional supporting information may be found in the online version of this article at the publisher's web-site IntroductionMonocytes (Mos) are large circulating leukocytes of the myeloid lineage that mediate essential functions of innate immunity including phagocytosis and cytokine production [1,2]. Mos are produced Correspondence: Dr. Luciana Balboa e-mail: luciana_balboa@hotmail.com in the bone marrow and released into the blood, and circulate in blood or reside in a spleen reservoir and, upon tissue infiltration they can differentiate into macrophages (M s) or DCs. Although human blood Mos are a highly heterogeneous population, two main subsets have been described: CD14 + CD16 − "classical" Mos are the major Mo population and CD14 + CD16 + Mos, the minor one (5-10% of total Mos). The latter subset has a more mature phenotype and an enhanced proinflammatory activity [3]. This heterogeneity has led to the hypothesis that Mos are committed to specific functions prior to tissue infiltration [4]. Also, C 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.eji-journal.eu 336Luciana Balboa et al. Eur. J. Immunol. 2013. 43: 335-347 an imbalance in the relative proportions of the subsets during acute inflammation [5] and several infectious diseases [6][7][8][9][10] including tuberculosis (TB) [11,12] has been described. TB, a disease that mainly affects the respiratory system, infects onethird of the world's population and kills 2 million people each year [13]. The success o...

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