T- cells differentiation studies
3.5 Evolutionary conservation of ‘viability recognition’
Detection of microbial viability by the innate immune system seems to be applicable across a wide range of bacterial species. Moreover, it regulates central components of the innate immune response, constituting a critical innate immune checkpoint to scale the infectious threat level posed by a given microbial encounter3. Consequently, we next sought to investigate the evolutionary conservation of ‘viability sensing’ on the host side.
As previously described, murine innate immune cells actively distinguish between living and dead microbes via the selective sensing of bacterial mRNA, a so-called
vita
-PAMP, which leads to the activation of the NLRP3 inflammasome and secretion of IL-1β99. In contrast,MHC-II CD40 CD80
Events
OX40L
Ctrl EC HKEC
ICOSL
Figure 9. Expression of activation markers and costimulatory molecules upon ‘viability sensing’.APCs were treated as in Fig. 5 and surface expression of the indicated markers was measured by flow cytometry at 18h post infection (n=5 donors).
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transcriptional responses, including TNF and IL-12 production, are largely independent of
vita
-PAMPs in murine APC.To study the evolutionary conservation of this process beyond human and laboratory mice, we choose to examine two very distant species, which in the past years have gained considerable interest: outbred farm swine,
Sus scrofa domestica
, and the Atlantic salmon,Salmo salar
. The former constitutes an appealing model for pharmaceutical and infection biology studies due to well-known physiological similarities with humans196, 197. Whereas interest in the immune system development and functions of fish, and salmon in particular, is fuelled by evolutionary studies198-200, as well as the growing demand for effective vaccines for the rapidly growing fish farming industry.
Sus scrofa domestica
Spleen samples were obtained from eight-weeks to one-year old domestic farm pigs,
homogenized and, subsequently, porcine monocytes (defined as lin-CD14+CD172+ cells) and dendritic cells (DC, defined as lin-CD14-CD172+ cells)201 were sorted by flow cytometry from total splenocytes preparations (Fig. 10). Monocytes and DC were stimulated with either live or heat-killed
ThyA
¯E. coli
(EC and HKEC) or with an attenuated, adenine- and histidine auxotrophic vaccine strain ofSalmonella enterica
serovar Typhimurium202, either live or heat-killed (ST and HKST respectively). Similar to human APC, porcine monocytes and DC produced large amounts of IL-12 selectively in response to living bacteria (EC or ST), but nota. b.
Figure 10. Isolation of porcine monocytes and dendritic cells. (a) Schematic of the methodology used to process porcine spleen sections to obtain first a splenocyte suspension and, subsequently, highly purified cell populations via flow cytometry-based sorting. (b) Representative FACS plot of the sorting strategy used for the isolation of porcine monocytes (CD14+CD172+) and dendritic cells (CD14-CD172+) from the spleen of pigs.
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in response to their respective killed counterparts (HKEC or HKST). Contrariwise, IL-6 was secreted regardless of bacterial viability (Fig. 11). Despite being constantly recorded, induction of IL-12 selectively in response to live EC and ST, compared to their killed counterparts, did not reach statistical testing significance. This was due, most probably, to the limited sample size, and its high inter-experimental and inter-sample variability (in terms of sex and age of the individual animals). Both monocytes and dendric cells were also highly responsive to TLR8 synthetic ligand CL075
Salmo salar
Leukocytes were isolated from peripheral blood of Atlantic salmon by discontinuous gradient centrifugation and monocytes were subsequently isolated by adherence (Fig. 12). Adherent cells Figure 11. Response to microbial viability in porcine APCs. Porcine monocytes (a) and dendritic cells (b) isolated from pig spleens by FACS were stimulated either with culture medium (ctrl), EC, HKEC, live attenuated S. enterica serovar Typhimurium (ST) or heat-killed ST (HKST) at a MOI of 10. IL-12p40 and IL-6 secretion in the culture supernatants was monitored by multiplex bead array at 24h post treatment (n=3 pigs for IL-6 and n=2 pigs for IL-12p40). Bars are mean + SEM. n.s.=non-significant (one-way ANOVA with post-hoc correction for multiple comparisons).
ctrl EC
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have been previously characterized as monocyte-/macrophage like cells203. Hence, adherent cells were stimulated with either control media, viable
E. coli
or heat-killedE. coli
.Interestingly also in salmon, monocytes/macrophages clearly discriminated bacteria according to their viability status as evidenced by the selective induction of proinflammatory cytokines TNF-α and IL-1β only in response to living bacteria, while IL-6 and the regulatory cytokine IL-10 are expressed in response to either bacterial stimulus (Fig. 13).
Three paralogs of IL-12p40 are present in the salmon genome, IL-12p40-b1, IL-12p40-b2 and IL-12p40-c respectively204, which most likely arose during the four rounds (4R) of whole genome duplication (WGD) experienced by the salmonid lineage compared to other vertebrates205. Of the three identified IL-12p40 genes two are induced specifically in response to infection with live bacteria, while the third isoform does not show a viability-dependent pattern of expression (Fig. 13). This differential modulation reflects what has been previously observed with the three IL-12 paralogs in response to PAMP204 stimulation and viral infection206. Collectively, these results indicate that the mechanism of microbial ‘viability recognition’ is likely largely conserved and a general feature of the vertebrate innate immune system being present even in teleost fishes, such as salmon, despite approximately 400 to 450 million years of Figure 12. Monocyte/macrophage-like cells isolated from Atlantic salmon. (a) Schematic depiction of the isolation strategy used to collect salmon monocytes. Leukocytes were isolated from blood sampled from the posterior vena caudalis of salmons by gradient centrifugation, plated and non-adherent cells were removed after 3h incubation. (b) Inverted microscopy pictures of adherent monocytic cells (20x magnification). (c) Cytospin preparation of representative cells. Pictures are captured with a 63x objective. Scale bar: 10 µm.
adherent monocytic cells 3h
gradient isolation blood sampling
a.
b. c.
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evolution separating them from mammals207, 208 and the differing environmental niches occupied.
Interestingly, also in