EXPERIMENTAL AND CLINICAL TRANSPLANTATION

Myeloid-derived suppressor cells (MDSCs) are a heterogenous myeloid-origin population of cells that potentially suppress immunological rejection in the field of transplantation.¹ There seem to be several reasons as to why MDSCs work. However, there is as yet no published study that has investigated the effect of MDSCs on intragraft donor-specific antibodies (DSA) accumulation, which is closely related to antibody-mediated rejection (ABMR).^2,3 Although it is important to independently evaluate ABMR from cellular rejection, the assessment of intragraft DSA has not been fully established in a mouse model. Thus, we established a solution-based assay (immunocomplex capture fluorescence analysis [ICFA]) for accurate detection of intragraft DSA in a mouse model.⁴ By using this method and a mouse cardiac transplant model, we assessed the role of MDSCs in DSA production and its accumulation in the allografts. The indexes of serum and intragraft DSA were examined according to our previous report.⁴ In brief, mean fluorescence intensity (MFI) of samples was measured by the Luminex system. Then, index = (X_H2 - (N_H2 – N_BB)X_BB/N_BB)/X_BB was calculated in order to compensate the baseline reaction, where X_H2 is the sample MFI, N_H2 is the MFI of the negative samples, N_BB is the MFI of blank beads of negative samples, and X_BB is the MFI of the sample blank beads. As negative controls, the indexes of control syngeneic cardiac grafts and the associated serum crossmatch show 0.97 ± 0.03 and 0.78 ± 0.50, respectively. In this experiment, the index ≥1.5 was determined as a positive result. All experiments were conducted according to our institutional animal care protocol.

A recipient C3H mouse (H2-K^k) received a cardiac graft from a Balb/C mouse (H2-K^d) (Figure 1A). In the untreated group, both class I and class II DSA production in serum were confirmed on postoperative day 10 (POD10). By POD10, intragraft class I DSA deposition was clearly observed, whereas class II DSA started to accumulate. The indexes of intragraft of both class I and class II DSA increased toward POD21 (Figure 1B). On the other hand, 2.0 × 10⁶ monocytic MDSCs (Ly6G^-/Ly6C⁺/CD11b⁺), which were isolated from the spleens of rapamycin-treated C3H recipient mice (3 mg/kg intra-peritoneally on POD0, POD2, POD4, and POD6), were higher on POD7 than 88% purity for use as a cell therapy (Figure 1A). These MDSCs were transferred via transcoronary artery just before reperfusion, according to our previous report,⁵ and intravenously on POD2. As a result, the indexes of both class I and class II DSA in serum were significantly lower on POD10. Subsequent intragraft DSA deposition was also notably suppressed. On POD21, the gradual accumulations of both class I and class II DSA in serum and intragraft class I DSA deposition were confirmed, although intragraft class II DSA deposition was not clearly confirmed in the MDSC transfer group (Figure 1B). Taken together, the transfer of MDSCs may postpone DSA production, although this effect is not perpetuated. Consequently, the graft survival was slightly prolonged (8.1 ± 0.9 vs 24.1 ± 3.8 days; P < .0001). These findings suggest that MDSCs also have a potential to regulate ABMR and result in a prolonged graft survival.

Isolated assessment of ABMR seems to be difficult on several occasions. Given the early production of DSA, this transplant model in particular shows cellular rejection and ABMR mixed influences. Therefore, independent intragraft DSA detection is a feasible method to assess more precisely ABMR. This tool could facilitate research on ABMR. Furthermore, ABMR potentially leads to graft failure without effective therapeutic modalities. In such a situation, the transfer of MDSCs may become an option as a cell therapy to ameliorate ABMR, provided that disadvantages of MDSCs, such as over immunosup-pression or onset of malignant tumors, are sufficiently controlled.

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