Mycobacterium leprae alters classical activation of human monocytes in vitro

Background Macrophages play a central role in the pathogenesis of leprosy, caused by Mycobacterium leprae. The polarized clinical presentations in leprosy are associated with differential immune activation. In tuberculoid leprosy, macrophages show a classical activation phenotype (M1), while macrophages in lepromatous disease display characteristics of alternative activation (M2). Bacille Calmette-Guérin (BCG) vaccination, which protects against leprosy, can promote sustained changes in monocyte response to unrelated pathogens and may preferentially direct monocytes towards an M1 protective phenotype. We previously reported that M. leprae can dampen the response of naïve human monocytes to a strong inducer of pro-inflammatory cytokines, such as BCG. Here, we investigated the ability of the pathogen to alter the direction of macrophage polarization and the impact of BCG vaccination on the monocyte response to M. leprae. Findings We show that in vitro exposure of monocytes from healthy donors to M. leprae interferes with subsequent M1 polarization, indicated by lower levels of M1-associated cytokine/chemokines released and reduced expression of M1 cell surface markers. Exposure to M. leprae phenolic glycolipid (PGL) 1, instead of whole bacteria, demonstrated a similar effect on M1 cytokine/chemokine release. In addition, we found that monocytes from 10-week old BCG-vaccinated infants released higher levels of the pro-inflammatory cytokines TNF-α and IL-1β in response to M. leprae compared to those from unvaccinated infants. Conclusion Exposure to M. leprae has an inhibitory effect on M1 macrophage polarization, likely mediated through PGL-1. By directing monocyte/macrophages preferentially towards M1 activation, BCG vaccination may render the cells more refractory to the inhibitory effects of subsequent M. leprae infection.


Background
Macrophages play a central role in the pathogenesis of leprosy, a chronic debilitating disease caused by Mycobacterium leprae. Monocyte/macrophages show a dynamic plasticity, allowing them to respond to environmental stimuli by presenting a classical (M1) or alternative (M2) activation state [1]. M1-activated macrophages release high levels of pro-inflammatory cytokines with enhanced microbicidal activity; M2 macrophages produce inhibitory cytokines and are less responsive to stimuli [2,3]. Leprosy presents as a spectrum of clinical manifestations, associated with differential immune activation. While tuberculoid leprosy shows robust cell-mediated immunity with predominantly M1-activated macrophages, lepromatous disease is characterized by strong humoral immunity and macrophages show an M2 phenotype [4,5].
Bacille Calmette-Guérin (BCG) vaccination protects against leprosy and is associated with reduced burden of unrelated diseases, suggesting non-specific protection that may involve shaping innate immunity [6][7][8]. BCG vaccination has been shown to induce sustained changes in the phenotype of circulating monocytes, with greater pro-inflammatory cytokine production [9]. Moreover, ex vivo stimulation of peripheral blood mononuclear cells (PBMC) from 10-week old infants vaccinated at birth with BCG, revealed a gene expression signature similar to an M1 macrophage profile with down-regulation of M2-associated genes [10].
We previously reported that stimulation of naïve monocytes from healthy donors with M. leprae alone, or M. leprae followed by BCG, induced the release of cytokine/chemokines that are associated with negative regulation of inflammation [11]. M. leprae itself was a poor inducer of the pro-inflammatory cytokine TNF-α, consistent with other reports [12]. However, when naïve monocytes were first stimulated by BCG and then exposed to M. leprae, the cells produced a pro-inflammatory cytokine profile matching that of BCG alone [11]. Here, we investigated the ability of M. leprae to interfere with M1 maturation of monocyte induced by exposure to IFN-γ and M2 maturation induced by exposure to IL4/ IL13 in vitro. We also tested whether BCG vaccination, by favoring an M1 phenotype, may render the cells resistant to the inhibitory effects of M. leprae.

Reagents
Mycobacterium leprae Thai-53 from the National Hansen's Disease Programs Laboratory Research, Louisiana State University, Baton Rouge (American Leprosy Missions and the Society of St. Lazarus) [13][14][15] and BCG from Trudeau Institute (Mycobacterial Culture Collection No. 1011) were prepared as described [11]. Purified M. leprae PGL-1, obtained through the NIH Biodefense and Emerging Infections Research Resources Repository NIAID, NIH: NR-19342, was used as described [16]. Peripheral blood mononuclear cells (PBMCs) from 10-week old infants unvaccinated (N = 18) or BCG-vaccinated at birth (N = 20) were provided by Dr. Willem Hanekom, University of Cape Town, South Africa.
These studies were approved by the Institutional Review Boards of Rutgers University and the University of Cape Town (Pro2012001418, Pro0120110233).
M1 polarization also resulted in significantly increased percentages of cells expressing the surface markers CCR7 and CD40, relative to unstimulated/untreated controls. When monocytes were pre-exposed to M. leprae at low MOI, the percentages of CCR7 + (P ≤ 0.001) and CD40 + (P ≤ 0.05) cells were reduced compared to M1 polarization alone (Fig. 2). Pre-stimulation with M. leprae at higher MOI did not result in further reduction (data not shown). HLA-DR + , CD80 + and CD86 + M1 cell percentages were unaffected by M. leprae pre-stimulation. The mean fluorescence intensities (MFI) of CCR7, CD40 and CD80 in M1 cells were also significantly reduced by M. leprae pre-exposure (Table 1). M. leprae alone was comparable to unstimulated/untreated controls.
In contrast, the impact of M. leprae on expression of M2 macrophage markers was minimal. Pre-stimulation with M. leprae increased the MFI of CD23 (295 ± 116 and 388.5 ± 153 at MOI 5:1 and 20:1, respectively) over M2 polarization alone (242.7 ± 108), with low levels produced by unstimulated/untreated cells (24.9 ± 12.3), and had no effect on IL-1Ra and IL-10 (data not shown). Thus, the effect of M. leprae on monocytes is primarily due to inhibition of M1 activation and does not appear to significantly affect M2 polarization.
Finally, we compared the effect of M. leprae on monocytes from 10-week old unvaccinated or BCG-vaccinated infants (Fig. 3). Levels of TNF-α and IL-1β released in response to M. leprae were significantly higher in monocytes from vaccinated infants than those from unvaccinated infants, while IL-6 and MCP-1 showed trends towards higher levels in vaccinated versus unvaccinated infants. These results demonstrate that in vivo activation of monocytes due to BCG vaccination may render the cells refractory to the inhibitory effects of M. leprae.

Conclusion
Exposure to M. leprae can alter the functional capacity of monocytes, which may diminish the efficacy of the host response to subsequent stimuli. Our results support increasing evidence suggesting that the innate immune response may be shaped by prior history of exposure, which could also explain the protection afforded by BCG vaccination against M. leprae and other unrelated pathogens.  The values represent the mean MFI ± Sem of 4-7 independent experiments (independent donors) for each condition. The statistical significance is shown as compared to the M1 cells. Lep5 (M. leprae MOI 5:1); Lep20 (M. leprae MOI 20:1).*P ≤ 0.05; **P ≤ 0.001 and ***P ≤ 0.0001 Fig. 3 Differential TNF-α and IL-1β response to M. leprae stimulation of monocytes isolated from 10-weeks old BCG-vaccinated and unvaccinated infants. Monocytes were isolated from PBMCs of 10-week old infants, unvaccinated or BCG-vaccinated at birth, and stimulated for 24 h with M. leprae. Data are expressed in pg/ml minus the value of the corresponding unstimulated controls. A 2-tailed paired t-test was used for statistical analysis between cells stimulated with M. leprae. *P ≤ 0.001; δ P ≤ 0.05