THP-1 cells (ATCC; no. TIB-202), derived from peripheral blood of an acute monocytic leukemia patient, were used as monocyte model. In addition, control PBMCs were isolated from apheresis products (Healthy 1, 2, 3) or full blood (Healthy 4-13) supplied by the blood bank Sanquin (Nijmegen) and CDG blood samples were collected from patients visiting the Radboudumc academic hospital (Nijmegen). The study was approved by the Institutional Review Board at Radboudumc (CMO-dossier number 2019-5654). Blood samples were taken on clinical indication and a part was used for this study after informed consent from either the patient or their legal representative, in accordance with the Declaration of Helsinki.

THP-1 cells and PBMCs were cultured in RPMI-1640 medium containing 25 mM HEPES (Gibco) and supplemented with 10% Fetal Bovine Serum (FBS) (Gibco) and 2 mM Ultraglutamine (Lonza). HEK293-FT cells (ATCC; no. PTA-5077), used for lentiviral production, were cultured in DMEM (Thermo Fisher Scientific) supplemented with 10% FBS (Gibco), 2 mM Glutamax (Gibco) and 1 mM Sodium Pyruvate (Gibco). All cell lines were cultured at 37°C in a humidified CO₂ (5%) incubator. For the shRNA expressing cell lines 2 µg/mL of puromycin was added to the medium to maintain selection pressure.

Cells were seeded at a density of 0.2 million/mL for control cells and 0.3 million/mL for tunicamycin (TM) treated cells, to correct for differences in proliferation rate between treatment groups. A concentration of 10 ng/mL TM (Abcam, stock 5 mg/mL in DMSO) or 0.0002% DMSO was used for the TM treatment and the control respectively, unless stated otherwise. After two days of culturing the cells were harvested, counted and reseeded at the starting density in fresh medium containing the relevant components. At day four of culturing the cells were harvested and used for experiments.

Lentiviral shRNA expression constructs specific for human PMM2 transcripts were purchased as glycerol stocks (TRC clone ID: TRCN0000300512 and TRCN0000300514; Sigma-Aldrich). Bacterial cultures were grown from individual colonies and plasmids were isolated using a Plasmid Midiprep Kit (Qiagen). Lentiviral packaging was performed in HEK293-FT cells using the Virapower Lentiviral Expression system (Invitrogen). The day before transfection 3 million cells were seeded in a 10 cm petri dish in 10 mL of medium. The next day, 9 µg of ViraPower packaging mix and 3 µg of PMM2 shRNA expression plasmid (or the Mission pLKO.1-puro Non-Target shRNA control; Sigma-Aldrich) were dissolved in 1.5 mL Optimem (Gibco). In a separate tube 36 µL Lipofectamine 2000 was diluted in 1.5 mL Optimem. Tubes were incubated for 5 m at room temperature whereafter plasmid and lipofectamine were mixed and incubated for another 20 m. Medium in the petri dish with HEK293-FT cells was replaced with 5 mL of fresh medium before the transfection mixture was gently added to the cells. The next day, medium was replaced with 10 mL fresh medium. Virus-containing supernatant was harvested 72 h after transfection and stored at -80°C until used.

For transduction purposes 1.2 million THP-1 cells were seeded in 2 mL of lentivirus-containing medium. Supernatant from both PMM2 shRNA expression vectors TRCN0000300512 and TRCN0000300514 were used in combination (1:1 ratio). Cells were incubated overnight and the next day virus supernatant was removed and replaced by fresh medium. Two days later, 2 µg/mL of puromycin was added to select for transduced cells. Selective pressure was maintained during all subsequent culturing steps. Individual clonal lines were derived via limiting dilutions and screened for PMM2 knockdown using qRT-PCR. Six to seven weeks after transduction the clonal derivatives were frozen and kept in liquid nitrogen for later use in experiments.

PBMCs were isolated by density gradient centrifugation. Blood in a EDTA collection tube was transferred to a 50 mL tube and the volume was adjusted to 35 mL with PBS. A layer of 14 mL lymphoprep (Stem Cell Technologies) was added under the diluted blood layer and the tube was centrifuged at 600 G for 30 m at room temperature with the brake off. The interphase layer containing the PBMCs was collected into another 50 mL tube and supplemented with cold PBS. The cells were centrifuged at 4°C for 15 m and this wash step was repeated two more times. The PBMCs were frozen and stored in liquid nitrogen until used in experiments.

Some 50,000-200.000 cells were seeded in a round bottom 96 well plate which was kept on ice during the staining procedure. All centrifugation steps were done at 360 G for 2 m at 4°C. Cells were washed once in PBS and twice in carbo-free blocking solution (Vector Laboratories). Subsequently, the cells were stained with either Concanavalin A-biotin (50 µg/mL; Thermo Fisher Scientific), MAA-biotin (EY laboratories, 50 µg/mL), MALII-biotin (Vector laboratories, 30 µg/mL) or SNA-FITC (Vector laboratories, 30 µg/mL) in Carbo-free buffer supplemented with 1 mM CaCl₂ and 1 mM MgCl₂ for 30 m (FITC-labelled lectins) or 45 m (biotin-labelled lectins). After staining, the cells were washed three times in PBA (PBS with 1% FCS). Cells stained with biotin-labelled lectins were treated with streptavidin 5 µg/mL Alexa 488 (Thermo Fisher Scientific) for 10 m. Then the cells were washed three times in PBA and fluorescent intensity was measured immediately by flow cytometry. For the TM experiments the cells were stained just before measurement with 1 µg/mL propidium iodide (Miltenyi Biotec) as a viability dye. Alternatively the cells were washed in PBS and fixed in 2% PFA for 30 m. Fixative was washed out three times with PBS and the cells were kept in the fridge until measured by flow cytometry within one week of staining. Fixed cells from TM experiments were stained with Fixable Viability dye eFluor 780 (eBioscience, 2000x diluted) before lectin staining. PBMCs were stained with CD14 antibody (Thermo Fisher Scientific, clone 61D3, 5 µg/mL in PBA) for 20 m after lectin staining and the secondary antibody goat-anti-mouse H&L A647 (Thermo Fisher Scientific, 2 µg/mL) was added at the same time as streptavidin and incubated for 20 m. Samples from TM treatment experiments were measured on a MACSquant analyzer or a CyAn ADP analyzer while the shRNA and PBMC samples were measured on a BD FACSlyric flow cytometry system. Data were analysed using FlowJo10 software. Median fluorescence intensity (MFI) values were normalized to the average MFI (set as 1) of the control. Before normalization, the MFI values of the negative controls were subtracted. Percentage positive cells was quantified by gating the population with higher fluorescence intensity than the negative (unstained) control. The percentage obtained in the negative control samples was subtracted (varying between 0-17% per sample).

For RNA isolation between 0.5 and 2 million cells were harvested by centrifugation and washed twice in PBS. The cell pellet was lysed in 350 µL RNA lysis solution and the suspension was put through an 18 gauge needle twelve times. The RNA lysate was either stored at -80°C until further processing or used for RNA isolation immediately. RNA was isolated with the Aurum Total RNA mini kit (Bio-Rad) and DNA contamination was removed with DNase I treatment. From the isolated RNA, 500 ng was used for cDNA synthesis with the iScript cDNA synthesis kit (Bio-Rad) in a total reaction volume of 20 µL. For each cDNA synthesis reaction a control without reverse transcriptase and a control without RNA template was taken along.

For quantitative RT-PCR, cDNA was diluted (80 times for ATF4, HSPA5 and DDIT3 and 25 times for PMM2) and 3 µL was mixed in a final volume of 10 µL containing 5 µL iQ SYBR Green Supermix (Bio-Rad) and 4 pmol of each primer. Primers used: ACTB Fw 5’-CGGGCCGTCTTCCCCTCCAT-3’ Rv 5’- TGGGCCTCGTCGCCCACATA-3’ (26), GAPDH Fw 5’-CCCGCTTCGCTCTCTGCTCC-3’ Rv 5’-CCTTCCCCATGGTGTCTGAGCG-3’ (26), ATF4 Fw 5’-GTTCTCCAGCGACAAGGCTA-3’ Rv 5’- ATCCTGCTTGCTGTTGTTGG-3’ (27), HSPA5 Fw 5’-TGTTCAACCAATTATCAGCAAACTC-3’ Rv 5’-TTCTGCTGTATCCTCTTCACCAGT-3’ (28), DDIT3 Fw 5’-ACCTCCTGGAAATGAAGAGGAAG-3’ Rv 5’- CAGTCAGCCAAGCCAGAGAA-3’ (29) (Sigma-Aldrich). For PMM2 knockdown measurement the Hs_PMM2_1_SG (QuantiText) primer assay was used in a 10 times dilution as recommended by the manufacturer. Samples were measured using a CFX96 Real-time System (Bio-Rad) set up to perform an initial 10 m at 95°C followed by 40 cycles of 15 s at 95°C and 50 s at 60°C. At the end a melt curve was made by increasing the temperature from 65°C-95°C at 1°C/s to confirm single product amplification.

One million PBMCs or 0.1 million THP-1 cells were added to a V-bottom plate together with 2 million Alexa488 labelled zymosan A from Saccharomyces cerevisiae (Sigma-Aldrich) or with heat-killed Candida albicans in 20 µL of serum-free RPMI-1640 medium. After incubation at 37°C for the indicated timepoints, cells were gently transferred from the V-bottom plate to a poly-L-lysin coated coverslip and fixed in 4% PFA in 0.1 M sodium phosphate buffer (PB buffer) for 30 m. The fixation as well as all the steps of the staining procedure were performed at room temperature. Samples were washed twice with PBS and blocked in blocking buffer (PB buffer supplemented with 20 mM glycine) for 60 m. Thereafter, cell membranes were stained for 1 h using 5 µg/mL LFA-1 antibody (Clone TS2/4, hybridoma) or CD14 antibody (clone 61D3; Thermo Fisher Scientific) in blocking buffer. Samples were washed three times in PBS and stained with a secondary goat-anti-mouse-H&L Alexa647 antibody (Thermo Fisher Scientific) in blocking buffer for 60 m. Finally, samples were washed twice in PBS and once in PB buffer before being sealed in Mowiol and imaged on a Leica DMI6000 widefield microscope using a HCX PL APO 63X/1.40 oil Leica Microsystems objective.

PBMC samples were thawed on the day of the assay. The staining and flow cytometry analyses was done as described previously using the ‘General panel’ from Aguirre-Gamboa et al. (30). In brief, PBMCs were stained in 25 µL of surface staining master mix containing CD14-ECD (clone RM052; Coulter) and CD16-FITC (clone 3G8; Coulter). Cells were washed twice and resuspended in 100 µL PBS containing 0.2% BSA. Then, cells were analysed using a 10-color Navios flow cytometer (Beckman Coulter) equipped with three solid-state lasers (488 nm, 638, and 405 nm). Data were analysed using Kaluza software version 1.3 (Beckman Coulter). CD14 positive cells were characterized as monocytes and subpopulations were identified by CD14 and CD16 expression.

For the PMM2 enzyme activity assay, 2.5 million cells were harvested and washed four times in PBS. Cell pellets were homogenized in a buffer containing 20 mM HEPES pH 7.1 (Sigma-Aldrich), 25 mM KCl, 1 mM dithiothreitol (Sigma-Aldrich), 10 µg/mL leupeptin (Roche), and 10 µg/mL antipain (FLUKA). Cells were disrupted by sonification and samples were centrifuged at 1550 G for 8 m and resulting supernatant was stored at -80°C until the enzyme activity assay was performed.

Supernatants were defrosted and centrifuged at 9000 G for 5 m. Total protein concentration was determined and PMM2 enzyme activity was measured spectrophometrically every 4.5 m at 30°C by the reduction of NADP to NADPH causing a change in A₃₄₀. All of the reagents needed to convert mannose 1-phosphate to gluconate 6-phosphate and NADPH were added to the samples except for the enzyme measured in the assay, namely PMM2. The reaction solution consisted of 50 mM HEPES pH 7.1, 5 mM MgCl₂, 2 µg/mL phosphoglucose isomerase (Roche), 1.7 µg/mL phosphomannose isomerase (Sigma-Aldrich), 10 µg/mL glucose 6-phosphate dehydrogenase (Roche), 1 µM mannose 1,6-bisphosphate, 0.25 mM NADP (Roche), and 0.34 mM mannose 1-phosphate (Sigma-Aldrich). As a negative control Mannose-1-phosphate was omitted from the sample and the extinction resulting from this was subtracted from the extinction measured in the sample in presence of the substrate. 1 U enzyme activity corresponds to the formation of 1 µmol of NADPH per m. The enzyme activity is normalized to the total amount of protein present in the sample (31).

Data are represented as mean ± standard deviation. Statistical analyses were performed using Graphpad Prism software 9.2.0. Primary monocyte and shRNA expressing THP-1 cell data were analyzed using an unpaired Student’s t-test. TM experiments were analyzed using a paired Student’s t-test, pairing data from TM treated cells with their respective untreated control. Results from the lectin binding assay in TM treated cells were analyzed using a ratio paired Student’s t-test because of varying lectin binding in untreated controls. Statistical significance of the different categories (binding and Internalization + binding) in the internalization assay was calculated separately by a One-way ANOVA and multiple comparisons.

The CDG patients included show an even distribution in gender (Table 1). Most patients were adults (>18) in the range between 25-50 years old but two younger patients were also included (ages 4 and 12 years old). The p.R141H and p.F119L mutation in PMM2, which are frequently reported in literature, were shared by some of the patients (32). However each patient in this cohort showed a unique PMM2 genotype which included both heterozygous (4/7 patients) and homozygous mutations (2/7 patients). The overall disease severity as assessed by the patients’ clinician, varied from mild to very severe. Three out of seven patients had a history with recurrent or severe infections. For all the patients we performed a flow cytometry analysis of the monocyte subsets and whenever possible we determined the PMM2 residual enzymatic activity. Patient 1, 2 and 3 were chosen for a more detailed assessment of monocyte characteristics because they represent a broad range of age groups and the clinical symptoms were moderate to severe. In addition, patient 1 and 3 have a history of recurrent or severe infections while patient 2 has no noticeable immunological symptoms. This allows us to compare monocyte characteristics in patients with or without immune dysfunction.

For this study peripheral blood mononuclear cells (PBMCs), were isolated from blood of PMM2-CDG patients and healthy donors. To assess the abundance of different monocyte populations, monocytes were phenotypically divided in classical (CD14⁺⁺CD16^-), intermediate (CD14⁺CD16⁺) and non-classical (CD14⁺⁺CD16⁺) monocytes. Classical (CD14⁺⁺CD16^-) monocytes are most abundant in blood and are described to be highly phagocytic while non-classical (CD14⁺⁺CD16⁺) monocytes are specialized in complement and FcR-mediated phagocytosis as well as anti-viral responses. Intermediate (CD14⁺CD16⁺) monocytes form the transition state between classical and non-classical cells and function in antigen presentation (33). The abundance of subpopulations is expressed as a percentage of the total monocyte population (Figure 1A). As expected, most monocytes (around 80%) were of the classical (CD14⁺⁺CD16^-) subgroup, while the non-classical (CD14⁺⁺CD16⁺) and intermediate (CD14⁺CD16⁺) monocytes represented around 10% and 5% of the circulating monocytes respectively. There were no significant differences found in the abundance of monocyte populations between PMM2-CDG patient and healthy donors.

Next, we determined the global glycosylation level at the cell membrane of monocytes from healthy donors and PMM2-CDG patients. We performed a lectin surface binding assay using two fluorescently labeled lectins, namely Concanavalin A (ConA), which binds to α-D-mannose and α-D-glucose structures, and Maackia amurensis agglutinin (MAA), which binds to α2-3-sialylated glycans, after optimizing their concentration range using THP-1 monocytes (Supplementary Figure 1). PBMCs were double stained for the CD14 marker and with either ConA or MAA and analyzed by flow cytometry (Figures 1B–D; Supplementary Figure 2). Histogram plots of the fluorescence intensity show binding of ConA and MAA to monocytes of all the healthy donors and PMM2-CDG patients (Figure 1B). However not all monocytes in one sample bind to these lectins, as can be observed by the percentage positive cells (Figures 1C, D). Interestingly, in the ConA histogram plots two distinct monocyte populations become visible in patient 1 and 3 material, more than 40% show high ConA binding. In the patient 2 sample, however, only a single monocyte group is detected, which has a lower overall ConA binding compared to healthy donor material (Figure 1C). All the PMM2-CDG patient monocytes show a trend towards a higher MAA binding compared to healthy donor controls, which is most prominent in patient 1-derived cells (Figure 1D).

Since only minor differences in glycosylation levels were observed between healthy control monocytes and PMM2-CDG monocytes, we were interested in the PMM2 enzyme activity in immune cells isolated from PMM2-CDG patients. PMM2 enzyme activity was measured in leukocytes of five patients included in this study (Table 1 and Figure 1E). The PMM2 enzyme activity in healthy control leukocytes varied between 0.47-5.31 mU/mg total protein and was on average 2.5 mU/mg. In PMM2-CDG patients, however, the enzyme activity was severely reduced: in two patients the activity was undetectable, while in the other patients there was between 1.6 and 5.6% residual activity, compared to the average in healthy controls.

An important function of monocytes is patrolling the tissue for recognition and uptake of invading pathogens. We used fluorescently labeled heat killed C. albicans to compare the phagocytic capacity of healthy and PMM2-CDG monocytes. We incubated PBMCs with C. albicans for different periods of time (15 m and 60 m) and at either 4°C (to allow only binding) or 37°C (to allow binding and internalization). The samples were then washed and fixed, and the PBMCs were stained for membrane protein CD14, which reveals the cell membrane and functions as a monocyte marker. The interaction of the monocytes with the pathogen was visualized with fluorescent microscopy and the cells were manually counted and scored as either ‘bound’ or ‘internalized’, representing cells with C. albicans in close contact with the cell membrane or inside the cytosol respectively (Figure 2A).

After 15 m of incubation with C. albicans only a small percentage of monocytes had internalized the pathogen, varying between 0-20%, and there were no noticeable differences between healthy donor and PMM2-CDG monocytes (Figure 2B). In contrast, after 60 m of incubation a higher percentage of PMM2-CDG patient monocytes had taken up C. albicans compared to the healthy donor controls, even though pathogen binding was comparable. This increase in uptake was not dramatic but observed in all three patient cells and most striking in patient 3.

PMM2-CDG is a rare disease and fresh blood from these patients is not always available. To mimic the disease phenotype in a monocyte cell model we inhibited glycosylation via tunicamycin (TM) treatment in the human THP-1 monocytic cell line. TM blocks the first step in the formation of the lipid-linked oligosaccharide (LLO) needed for N-glycosylation by inhibiting GlcNAc phosphotransferase. For this reason TM is broadly used to study the effect of hypoglycosylation (34–36). Literature suggests that complete absence of PMM2 enzyme activity is not compatible with life and therefore some residual activity remains in all PMM2-CDG patients (32, 37). To simulate this aspect of PMM2-CDG we aimed to find the minimal dose of TM treatment of THP-1 cells that interfered with glycosylation but had a limited impact on cell viability. The optimal TM treatment of THP-1 cells was established to be four days of treatment with 10 ng/mL TM (Supplementary Figure 3). This treatment was used for the rest of this study and the effect of TM treatment on the viability, surface glycosylation level and ER stress level was assessed (Figure 3).

Although TM treatment increased the number of dead cells, the viability of the cells was still very high (Figure 3A).The membrane glycosylation was significantly altered by TM treatment as a 70% reduction in ConA and 50% reduction of MAA binding to the cell surface was observed (Figures 3B, C). In addition, binding of the cell to Sambucus nigra (SNA) lectin, which preferentially binds to α2-6-sialylated glycans, appeared to be lower, albeit not statistically significant, after TM treatment. To determine whether TM specifically affected N-glycosylation and not O-glycosylation, we determined THP-1 binding to the fluorescent Maackia amurensis lectin II (MALII), which binds to α2-3-sialylated O-glycans (38). No significant change in MALII binding between TM treated and control cells was observed, strongly suggesting that the TM treatment only affected N-glycosylation in THP-1 monocytes.

Since TM is a known ER stress inducer and glycosylation is important in protein folding and quality control, we determined to what extent TM treatment induced ER stress in THP-1 monocytes. To this aim, we measured RNA expression of three ER stress markers, ATF4, HSPA5 and DDIT3 by RT-qPCR (Figure 3D). RNA expression of all these markers was significantly increased after TM treatment compared to the control, the effect was strongest on DDIT3 expression where a 16-fold increase of RNA expression was observed in TM treated cells. To determine whether the ER stress measured in these TM treated THP-1 cells affected cell viability on a long term, we incubated THP-1 cells with different concentrations of TM for up to three weeks and found that more than 70% of the cells were still viable and maintained reduced N-glycosylation levels (Supplementary Figure 3). Based on these results, we established the conditions under which a minimum amount of TM was found to preserve viability, induce a tolerable ER stress level while significantly decreasing N-glycosylation in THP-1 monocytes.

As an alternative to TM treatment, we knocked down PMM2 expression in THP-1 cells using shRNAs via stable lentiviral transduction. First, we determined the reduction in PMM2 mRNA levels in these knockdown cells and found about 50% decrease (Figure 4A). To determine whether the shRNA-induced reduction in PMM2 mRNA indeed affected PMM2 activity, we prepared cell lysates and performed a PMM2 enzyme activity assay. We found a highly significant decrease in PMM2 enzymatic activity that went from 2.62 mU/mg in the non-targeting (NT) control cells to 0.37 mU/mg in PMM2 knockdown cells. This corresponds to about 14% residual enzymatic activity in PMM2 knockdown cells, which is in line with the residual enzymatic activity previously reported for CDG patient cells (Figure 4B) (31, 39). We next determined the overall glycosylation level by fluorescent lectin binding assays but, surprisingly, none of these lectins showed an altered binding in PMM2 knockdown cells compared to the NT cells (Figures 4C, D). We also measured ER stress in these PMM2 knockdown cells by analyzing the RNA expression levels of the ER stress markers, ATF4, HSPA5 and DDIT3, but these were not significantly impacted by the PMM2 knockdown (Figure 4E).

Collectively, these data suggest that PMM2 knockdown THP-1 cells may better resemble the primary monocytes of CDG patients than TM-treated monocytes.

To test the phagocytic function of glycosylation impaired THP-1 cells, we performed an internalization assay using fluorescent zymosan, cell wall particles derived from S. cerevisiae, and heat killed C. albicans as model pathogens. THP-1 cells were incubated with these fungal particles for different periods of time (15 m, 60 m or 90 m) at 4°C (to allow binding only) or 37°C (to allow binding and internalization). After fixation the cells were stained with an antibody against the leukocyte specific integrin LFA-1 to visualize the membrane by fluorescent microscopy. The binding and internalization events were manually counted as described for the internalization assay with PMM2-CDG patient monocytes (Figure 5A).

The binding and uptake of fluorescent zymosan particles by TM treated and PMM2 knockdown THP-1 monocytes were surprisingly quite different (Figures 5B, C). TM treated monocytes showed a very strong decrease in zymosan uptake compared to the untreated cells, even after 90 m of incubation at 37°C (Figure 5B), whereas PMM2 knockdown THP-1 monocytes showed a similar binding and uptake as the controls, i.e. cells treated with NT shRNAs (Figure 5C).

For the PMM2 knockdown THP-1 monocytes, phagocytosis of C. albicans was measured, using the protocol for the PMM2-CDG patient PBMCs (Figure 5D). With respect to zymosan, similar binding levels but clearly less uptake of C. albicans were observed. No differences were observed between NT and PMM2 knockdown THP-1 monocytes. When compared to the phagocytosis of C. albicans by the monocytes of PMM2-CDG patients (Figure 2), the PMM2 knockdown THP-1 monocytes hardly showed any uptake.

In summary, the glycosylation impaired THP-1 monocytes showed a very heterogeneous ability to internalize zymosan particles or C. albicans and neither the TM-treated nor the PMM2 knockdown monocytes were found to recapitulate the behavior of PMM2-CDG patient monocytes.