Macrophages, especially primary macrophages, are challenging to transfect as they specialize in detecting molecules of non-self origin. We describe a protocol that allows highly efficient transfection of primary macrophages with mRNA generated from DNA templates such as plasmids.
Macrophages are phagocytic cells specialized in detecting molecules of non-self origin. To this end, they are equipped with a large array of pattern recognition receptors (PRRs). Unfortunately, this also makes macrophages particularly challenging to transfect as the transfection reagent and the transfected nucleic acids are often recognized by the PRRs as non-self. Therefore, transfection often results in macrophage activation and degradation of the transfected nucleic acids or even in suicide of the macrophages. Here, we describe a protocol that allows highly efficient transfection of murine primary macrophages such as peritoneal macrophages (PM) and bone marrow-derived macrophages (BMDM) with mRNA in vitro transcribed from DNA templates such as plasmids. With this simple protocol, transfection rates of about 50-65% for PM and about 85% for BMDM are achieved without cytotoxicity or immunogenicity observed. We describe in detail the generation of mRNA for transfection from DNA constructs such as plasmids and the transfection procedure.
Macrophages are phagocytic cells that specialize in detecting, ingesting and degrading microbes, apoptotic cells and cellular debris. Moreover, they help to orchestrate immune responses by secreting cytokines and chemokines and by presenting antigens to T cells and B cells. Macrophages also play important roles in numerous other processes, such as wound healing, atherosclerosis, tumorigenesis and obesity.
To be able to detect non-self molecules such as pathogen-associated molecular patterns (PAMPs) and out-of-place molecules such as damage-associated molecular patterns (DAMPs), macrophages are equipped with a large array of pattern recognition receptors (PRR)1. Unfortunately, this also makes macrophages particularly challenging to transfect2 as the transfection reagent3 and the transfected nucleic acids4,5,6,7 often are recognized by the PRRs as non-self. For this reason, transfection of macrophages using chemical or physical methods8 usually results in macrophage activation and degradation of the transfected nucleic acids or even in macrophage suicide via pyroptosis, a form of programmed lytic cell death triggered after recognition of cytosolic PAMPs/DAMPs such as DNA or foreign RNA9. Biological transfection of macrophages using viruses such as adenoviruses or lentiviruses as vectors is often more efficient, yet construction of such viral vectors is time-consuming and requires biosafety level 2 equipment10,11.
Thus, although macrophages are the subject of intensive research, analysis of their functions on the molecular level is hampered because one of the most important tools of molecular biology, the transfection of nucleic acid constructs for exogenous expression of proteins, is hardly applicable. This often forces researchers to use macrophage-like cell lines rather than bona fide macrophages. Applications for nucleic acid construct transfection include expression of mutated or tagged protein versions, overexpression of a specific protein, protein re-expression in a respective knockout background and expression of proteins from other species (e.g., Cre recombinase or guide RNA and Cas9 for targeted gene knockout).
Here, we describe a protocol that allows highly efficient transfection of (usually hard to transfect) primary macrophages, that is murine peritoneal macrophages (PM) and bone marrow-derived macrophages (BMDM) with mRNA generated from DNA templates such as plasmids. Importantly, the in vitro transcribed mRNA generated using this protocol contains the naturally occurring modified nucleosides 5-methyl-CTP and pseudo-UTP that reduce immunogenicity and enhance stability4,6,7,12,13. Moreover, the 5'-ends of the in vitro transcribed mRNA are dephosphorylated by Antarctic phosphatase to prevent recognition by the RIG-I complex14,15. This minimizes innate immune recognition of the in vitro transcribed mRNA. With our easy to perform protocol, transfection rates between 50-65% (peritoneal macrophages (PM)) and 85% (BMDM) are reached while, importantly, there is no cytotoxicity or immunogenicity observed. We describe in detail (i) how the immunologically silenced mRNA for transfection can be generated from DNA constructs such as plasmids and (ii) the transfection procedure itself.
Here we present a protocol for highly efficient transfection of usually hard-to-transfect primary macrophages with in vitro transcribed mRNA. Importantly, transfection of the macrophages using this protocol does not induce cell death or activate proinflammatory signaling indicating that neither the transfection reagent nor the transfected mRNA are recognized as non-self.
The quality of the mRNA is of key importance for successful transfection of macrophages using this protocol. Thus, great car…
The authors have nothing to disclose.
This work was supported by the Deutsche Forschungsgemeinschaft (SFB 670).
5-methyl-CTP (100 mM) | Jena Biosience | NU-1138S | stored at -20 °C |
Antarctic phosphatase | New England BioLabs | M0289 | stored at -20 °C |
Antarctic phosphatase reaction buffer (10X) | New England BioLabs | B0289 | stored at -20 °C |
anti-NEMO/IKKγ antibody | Invitrogen | MA1-41046 | stored at -20 °C |
anti-β-actin antibody | Sigma-Aldrich | A2228 | stored at -20 °C |
Petri dishes 92,16 mm with cams | Sarstedt | 821,473 | stored at RT |
CD11b Microbeads mouse and human | Miltenyi Biotec | 130-049-601 | stored at 4 °C |
Cre recombinase + T7-Promotor forward primer | Sigma-Aldrich | 5′-GAAATTAATACGACTCACTATA GGGGCAGCCGCCACCATGTCC AATTTACTGACCGTAC-3´, stored at -20 °C |
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Cre recombinase + T7-Promotor reverse primer | Sigma-Aldrich | 5′-CTAATCGCCATCTTCCAGCAGG C-3′, stored at -20 °C |
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DNA purification kit: QIAquick PCR purification Kit | Qiagen | 28104 | stored at RT |
eGFP + T7-Promotor forward primer | Sigma-Aldrich | 5´-GAAATTAATACGACTCACTATA GGGATCCATCGCCACCATGGTG AGCAAGG-3´, stored at -20 °C |
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eGFP + T7-Promotor reverse primer | Sigma-Aldrich | 5´-TGGTATGGCTGATTA TGATCTAGAGTCG-3´, stored at -20 °C |
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Fast Digest buffer (10X) | Thermo Scientific | B64 | stored at -20 °C |
FastDigest XbaI | Thermo Scientific | FD0684 | stored at -20 °C |
high-fidelity polymerase with proofreading: Q5 High-Fidelity DNA-Polymerase | New England Biolabs Inc | M0491S | stored at -20 °C |
IKKβ + T7-Promotor forward primer | Sigma-Aldrich | 5′-GAAATTAATACGACTCACTATA GGGTTGATCTACCATGGACTACA AAGACG-3′, stored at -20 °C |
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IKKβ + T7-Promotor reverse primer | Sigma-Aldrich | 5′-GAGGAAGCGAGAGCT-CCATCTG-3′, stored at -20 °C | |
in vitro mRNA transcription kit: HiScribe T7 ARCA mRNA kit (with polyA tailing) | New England BioLabs | E2060 | stored at -20 °C |
LS Columns | Miltenyi Biotec | 130-042-401 | stored at RT |
MACS MultiStand | Miltenyi Biotec | 130-042-303 | stored at RT |
mRNA transfection buffer and reagent: jetMESSENGER | Polyplus transfection | 409-0001DE | stored at 4 °C |
Mutant IKKβ IKK-2S177/181E plasmid | Addgene | 11105 | stored at -20 °C |
Mutant NEMOC54/347A plasmid | Addgene | 27268 | stored at -20 °C |
pEGFP-N3 plasmid | Addgene | 62043 | stored at -20 °C |
poly(I:C) | Calbiochem | 528906 | stored at -20 °C |
pPGK-Cre plasmid | F. T. Wunderlich, H. Wildner, K. Rajewsky, F. Edenhofer, New variants of inducible Cre recombinase: A novel mutant of Cre-PR fusion protein exhibits enhanced sensitivity and an expanded range of inducibility. Nucleic Acids Res. 29, 47e (2001). stored at -20 °C | ||
pseudo-UTP (100 mM) | Jena Biosience | NU-1139S | stored at -20 °C |
QuadroMACS Separator | Miltenyi Biotec | 130-090-976 | stored at RT |
Rat-anti-mouse CD11b antibody, APC-conjugated | BioLegend | 101212 | stored at 4 °C |
Rat-anti-mouse F4/80 antibody, PE-conjugated | eBioscience | 12-4801-82 | stored at 4 °C |
recombinant M-CSF | Peprotech | 315-02 | stored at -20 °C |
RNA purification kit: MEGAclear transcription clean-up kit | ThermoFisher Scientific | AM1908 | stored at 4 °C |
RNAse-degrading surfactant: RnaseZAP | Sigma-Aldrich | R2020 | stored at RT |
ultrapure LPS from E.coli O111:B4 | Invivogen | stored at -20 °C | |
Wild type IKKβ plasmid | Addgene | 11103 | stored at -20 °C |
Wild type NEMO plasmid | Addgene | 27268 | stored at -20 °C |