Different responses of human anti-HLA and anti-αgal antibody to long-term intravenous immunoglobulin therapy

Concentrated human immunoglobulin (IVIG) has been administered intravenously in the treatment of autoimmune disorders and to reduce anti-HLA antibodies in highly sensitized patients awaiting organ transplantation. It has also been shown, in experimental animals, to prevent the hyperacute rejection of discordant xenografts, possibly by anticomplement activity. The aim of the present study was to assess the effect of IVIG therapy on both acquired anti-HLA antibodies and natural antigalactose α1–3 galactose (αGal) antibodies in five patients awaiting heart transplantation. Five patients placed on mechanical circulatory support who had developed high HLA panel-reactive antibodies (PRA) or in whom the percentage of PRA was increasing rapidly were treated weekly with 500 mg/kg IVIG, which contained 1% of anti-αGal IgG. Levels of PRA, anti-αGal IgG and IgM, and serum cytotoxicity to pig cells were measured before, during, and after therapy. PRA percentages in the five patients were initially 85%, 53%, 23%, 19% and 19% (mean 39%). Mean PRA fell by 66% after 3 months of therapy (to a mean PRA of 14%), and by 96% after 6 months therapy (to a mean PRA of 2%). Anti-αGal antibody levels and serum cytotoxicity to pig aortic endothelial cells did not change significantly. These results confirm the effectiveness of IVIG therapy in reducing PRA in HLA highly sensitized patients. It is likely that IVIG does not contain the relevant anti-HLA antibody, resulting in an accelerated catabolism of native alloantibodies. However, as IVIG contains a normal level of anti-αGal IgG, catabolism of anti-αGal IgG is not modified, as it is being continuously replaced. To achieve a decrease in the anti-αGal IgG level it would be necessary to use IVIG depleted of this antibody.     

Specific absorption of lymphocytotoxic alloantibodies by the liver in inbred rats

It has been suggested that liver allografts are less sensitive to lymphocytotoxic antibodies than other organ allografts. In this experimental study in sensitized inbred rat recipients, we have used extracorporeal liver hemoperfusion to study interactions between the liver and lymphocytotoxic antibodies. Donor-specific liver hemoperfusion can delay hyperacute rejection of heart allografts and reduce the level of lymphocytotoxic antibodies. Immunofluorescence examination of the hemoperfused liver revealed deposits of C3 on Kupffer cells and of IgG on sinusoidal cells. In control rats in which a third-party liver, a donor-specific splenic or renal hemoperfusion was performed, heart allograft survival was less prolonged. The decrease in antibody levels was not significant and the deposit of C3 and IgG was much less evident. Similarly, previous blockade of the Kupffer cells of the donor-specific hemoperfused liver by dextran sulfate suppressed the effect of liver hemoperfusion. These results support the hypothesis that resistance of the liver to hyperacute rejection might be due to a massive and nontoxic absorption of lymphocytotoxic antibodies onto nonparenchymal liver cells.     

Skewing of pretransplant anti-HLA class I antibodies of immunoglobulin G isotype solely toward immunoglobulin G1 subclass is associated with poorer renal allograft survival

Sensitized patients with lymphocytotoxic immunoglobulin (Ig)G anti-human leukocyte antigen (HLA) antibodies have an increased risk of rejection and poorer graft survival. Little is known, however, about the correlation between IgG antibody subclass and clinical outcomes. We identified 20 sensitized renal transplant recipients (panel reactive antibody >15%), all of whom had anti-HLA class I antibodies of an IgG isotype with known specificity before transplantation but who received a crossmatch negative graft. We analyzed the degree of skewing solely toward IgG1 (n=11) or to other IgG subclasses with or without IgG1 (n=9) and correlated these findings with graft survival. At last follow-up (median follow-up 28 months), 6 of 11 patients (55%) with anti-HLA antibodies skewed toward IgG1 had lost their grafts compared with 0 of 9 patients (0%) with anti-HLA antibodies not skewed toward IgG1 (P =0.01 log-rank test). Anti-HLA antibodies of an IgG1 subclass may be a novel marker predicting poor graft outcome.     

Desensitization strategies enabling successful renal transplantation in highly sensitized patients

Abstract:  Currently, the number of highly sensitized patients awaiting a renal transplant is increasing on the waiting lists of different organ exchange organizations. Due to the presence of antibodies against a broad variety of human leukocyte antigen (HLA) specificities, highly sensitized patients have a markedly reduced chance of receiving a crossmatch-negative organ. It has long been recognized that hyperacute rejection is associated with the presence of donor-specific anti-HLA antibodies at the time of transplantation. Meanwhile treatment protocols have been developed to achieve successful transplantation across antibody barriers. Therefore, the presence of donor-specific anti-HLA antibodies and a positive serological crossmatch are no longer considered as an absolute contraindication to renal transplantation. Mainly, two desensitization protocols have been established in order to overcome a positive crossmatch or to enhance the chance of highly sensitized patients to receive a crossmatch-negative organ: high-dose intravenous immunoglobulin (IVIg) or low-dose IVIg in combination with plasmapheresis. Herein, we summarize the characteristics of these two treatment regimes along with other alternative approaches that are currently used for the management of kidney graft recipients with broad alloantibody reactivity against potential kidney donors.     

Acute Humoral Rejection of Human Lung Allografts and Elevation of C4d in Bronchoalveolar Lavage Fluid

Antibody-mediated rejection is well established for renal allografts but remains controversial for lung allografts. Cardinal features of antibody-mediated rejection in renal allografts include antibodies to donor human leukocyte antigen (HLA) and evidence for antibody action, such as complement activation demonstrated by C4d deposition. We report a lung allograft recipient with circulating antibodies to donor HLA who failed treatment for acute cellular rejection but responded to therapy for humoral rejection. To address the second criteria for antibody-mediated rejection, we determined whether complement activation could be detected by measuring C4d in bronchoalveolar lavage fluid (BALF) by ELISA. Airway allergen challenge of asthmatics activates the complement pathway; therefore, we used BALF from asthmatics pre- and post-allergen challenge to measure C4d. These controls demonstrated that ELISA could detect increases in C4d after allergen challenge. BALF from the index patient had elevated C4d concomitant with graft dysfunction and anti-donor HLA in the absence of infection. Analysis of BALF from 25 additional lung allograft recipients showed that C4d concentrations >100 ng/mL were correlated with anti-HLA antibodies (p = 0.006), but were also observed with infection and in asyptomatic patients. The findings support the occurrence of anti-HLA-mediated lung allograft rejection and suggest that C4d measurement in BALF may be useful in diagnosis.     

免疫吸附预防高致敏肾移植受者超急性排斥反应

目的探讨免疫吸附对高致敏‘肾移植受者超急性排斥反应的预防作用。方法对10例群体反应抗体（PRA）〉40％的‘肾移植受者术前行免疫吸附治疗，观察其超急性排斥反应发生情况及不良反应。结果10例高致敏肾移植受者均未发生超急性排斥反应，仅有2例发生急性排斥反应，并通过免疫吸附及调整免疫抑制剂得到逆转。所有受者随访至今移植‘肾功能良好，未发生排斥反应。结论免疫吸附可以安全、有效地预防高致敏人群‘肾移植术后超急性排斥反应。     

Cold-reacting IgM antibody-induced renal allograft failure. Similarity to hyperacute rejection

Hyperacute rejection of renal allografts is usually mediated by IgG antibody, but recent studies indicate that cold-reacting IgM alloantibodies are also associated with immediate malfunction of renal allografts. We report 2 cases of cold-reacting IgM-mediated allograft malfunction in which immediate posttransplant biopsies resembled hyperacute rejection. This reaction can probably be prevented by warming the kidney just before transplantation.     

Monitoring anti-HLA Class I IgG antibodies in renal transplant recipients

Anti-HLA class I IgG antibodies play an important role in hyperacute rejection but the significance of its de novo appearance or increase in levels during the posttransplant period remains controversial. The purpose of this study was to determine the correlation between the anti-HLA class I IgG antibodies and posttransplant events during the first 4 months after renal transplantation. From 200 renal allograft recipients, 549 serum samples were retrospectively evaluated. Patients who experienced graft dysfunction confirmed by biopsy had three serum samples tested: before, during (within 24 hours), and after the event. The presence of anti-HLA antibodies was observed in recipients with chronic allograft nephropathy (60%); acute rejection (clinical criteria without biopsy 57.1%); rejection types IIA (7.1%), IIB (40%), and III (50%); borderline changes (42.8%); acute tubular necrosis (34.4%); infarction (25%); and no rejection (12.5%). We observed a high incidence of anti-HLA class I IgG antibodies during acute tubular necrosis, borderline changes, acute rejection types IIB and III, and chronic allograft nephropathy.     

Panel reactive antibody positivity and associated HLA antibodies in Turkish renal transplant candidates

Pre- and post-renal transplantation panel reactive antibody (PRA) screening is associated with increased incidence of hyperacute or acute graft rejection and graft loss. This study was designed to find any relationship PRA sensitization and associated human leukocyte antigen (HLA)-specific antibodies in Turkish renal transplant candidates. We included 340 patients who were in the renal transplantation waiting list in the study. We determined PRA sensitization ratio and the associated anti-HLA IgG antibody distribution of the patient group. The PRA testing was currently performed and levels above 30% were accepted to be positive. The PRA class I positivity was determined in 24 (7%) and class II in 34 (10%) of the patients. The most frequent HLA antibodies for class I were B56, A2, A34, A1, A23, A24 and B61; and for class II were DR11, DR14, DQ7, DR10, DQ5, DR1 and DR7, respectively. From these, the increase of the numbers of anti-HLA class II antibodies was significantly correlated with the increase of PRA sensitization ratio. In conclusion, the identification of the associated HLA-specific antibodies and correlation with the Turkish population HLA antigen distribution will identify the high-risk patients who are candidates for transplantation.     

Cross-reactivity between swine leukocyte antigen and human anti-HLA-specific antibodies in sensitized patients awaiting renal transplantation

Xenotransplantation is increasingly viewed as a promising way to alleviate the problem of patients who have alloreactive lymphocytotoxic antibodies and therefore tend to accumulate on the waiting list for renal transplantation. One barrier to xenotransplantation in these patients could be the hyperacute or acute vascular rejection as a result of preexisting anti-HLA antibodies that recognize swine leukocyte antigens. The cross-reactivity of sera from 98 patients with pig lymphocytes was studied by flow cytometry. After absorption of xenoreactive natural antibodies (XNA), isotype, class, and antibody specificity causing a positive cross-match (XM) were determined. For nonsensitized patients, all of the antibody binding to pig lymphocytes was due to XNA, which were removed by pig red blood cells absorption. In contrast, in sensitized patients, after removal of XNA, pig lymphocyte XM remained positive. There was no correlation between antibody binding to pig lymphocytes and Ig isotype (IgG or IgM) or HLA class-specific antibodies. For testing evidence that class II-specific antibodies were responsible for antibody binding to pig lymphocytes, HLA class I-specific antibodies were absorbed with pooled human platelets. It was confirmed that HLA class II-specific antibodies were responsible for the positive pig XM, but the strength of the positive XM was weaker than the strength caused by HLA class I-specific antibodies. Sera with multiple specificities (plurispecific sera) displayed a greater frequency of cross-reactivity with swine leukocyte antigens (P < 0.05). Seven of 11 highly immunized patients without cross-reactivity IgG with porcine lymphocytes showed positive XM before an IgM was used. The results demonstrate the cross-reactive nature of HLA antibodies and therefore point out the need to perform a prospective XM after absorption of XNA in presensitized individuals.     

Relationship between the liver and lymphocytotoxic alloantibodies in inbred rats. Specific absorption by nonparenchymal liver cells

Liver allografts have a privileged status with regard to hyperacute rejection. In this experimental study, we have used extracorporeal liver hemoperfusion in sensitized rats in order to analyze reactions between lymphocytotoxic antibodies and the liver. In sensitized BN rats, a donor-specific (Lewis) extracorporeal liver hemoperfusion can delay hyperacute rejection of heart allografts and reduce the level of lymphocytotoxic antibodies. The decrease in the level of antibodies could be due to massive absorption of antibodies by the liver or to release of major histocompatibility complex antigens in a soluble form. Immunofluorescent examination of the hemoperfused liver revealed important deposits of C3 on Kupffer cells and of IgG on sinusoidal cells. On the contrary, in control rats in which a third-party (DA) liver hemoperfusion was performed, heart allograft survival was less prolonged, the decrease in the level of lymphocytotoxic antibodies was not significant, and the deposits of IgG and C3 were much less evident. The level of circulating immune complexes was unchanged after a donor-specific or a third-party liver hemoperfusion. These results support the hypothesis that resistance of the liver to hyperacute rejection might be due to a massive and nontoxic absorption of lymphocytotoxic antibodies on nonparenchymal liver cells.     

动态分析HLA和MICA特异性抗体对移植肾功能的影响

目的 探讨肾移植前后人类白细胞抗原(HLA)和主要组织相容性一类相关链A基因(MICA)抗体特异性对移植后排斥反应和移植肾功能的影响. 方法采用免疫荧光液相芯片技术检测27例肾移植(尸供22例,亲属活体供肾5例)受者手术前后抗HLA抗体和MICA抗体的特异性和阳性值变化,并结合供受者的基因分型,区分供体特异性抗体和非供体特异性抗体.取同期的临床资料和SCr水平进行分析. 结果 27例移植患者中带肾存活26例,移植肾失功1例.移植后1、3、6、12个月时动态随访24例,失访2例.27例患者移植前预存抗体7例(25.9%),其中HLA抗体阳性2例、MICA抗体阳性3例,HLA和MICA抗体均阳性2例.肾移植前HLA和MICA抗体均阴性者中移植后3～6个月产生新生抗体3例.1例新生HLA-Ⅱ类特异性抗体者,移植半年后出现慢性排斥反应,经治疗术后1年SCr>200 btmol/L.3例肾移植前MICA抗体阳性者,术后MICA抗体的特异性均无改变,但抗体的阳性分值呈现2～8分的变化,1年后均升高到移植前(4～8分)水平.移植前预存低阳性率HLA-Ⅱ类特异性抗体者1例,移植后2周有发热等排斥反应,巨细胞病毒检测阳性,移植后1个月时SCr为171μmol/L,3个月升高到236μmol/L.24例分为抗体阴性组(14例)和抗体阳性组(10例).移植后1个月和1年时SCr水平2组间比较差异有统计学意义(P=0.03,0.05). 结论 移植后3～6个月是新生抗体变化的重要随访时间,可根据HLA和MICA抗体的特异性和阳性分值变化,尽早采取有效方法预防排斥反应和减少移植肾功能减退的发生和发展.     

The significance of the anti-class I antibody response. I. Clinical and pathologic features of anti-class I-mediated rejection

In renal transplantation, preformed cytotoxic antibody against donor HLA class I antigens causes hyperacute rejection of renal allografts, but its pathogenic significance when it develops in the posttransplant period is unknown. In the present studies we describe the clinical and pathologic features of patients with rejection associated with anti-class I. In the course of 400 consecutive cadaveric renal transplants, 7 patients were identified who had antibody against donor class I HLA antigens in association with atypical but distinctive patterns of rejection. All 7 were presensitized. In 3 patients, the transplant had been inadvertently performed with a positive donor-specific T cell crossmatch. In the remaining 4, the T cell crossmatch on current sera was negative but became positive posttransplant. The clinical picture was deterioration of graft function with rapid onset of oliguria, apparently due to acute tubular necrosis, but with persistence of blood flow demonstrable by radioisotope scan studies. Renal histology showed that the typical lesions observed in cell-mediated rejection, such as tubulitis and interstitial infiltration, were absent. Granular complement deposition (6), polymorphonuclear infiltration (6), and endothelial injury in the microvasculature (6) were common, and mononuclear infiltrates were absent (2) or not prominent (4). In 3 patients the glomerular changes resembled a picture of hemolytic uremic syndrome, with capillary fibrin thrombi and widening of the subendothelial space. IgG staining was negative. The pathologic features suggest that anti-class I antibody appearing or persisting in the early posttransplant period injures the endothelium of the microvasculature, with the clinical presentation different from that of hyperacute rejection. Particularly in sensitized patients, rapid deterioration in function, leading to a picture of acute tubular necrosis, with pathologic features of endothelial injury in the microcirculation, should suggest the diagnosis of anti-class I-mediated rejection.     

Effect of posttransplant anti-HLA antibody on outcome of renal transplantation

Pretransplant anti-HLA antibody has an impact on renal transplantation (RT) outcome. However, the role of posttransplant anti-HLA antibody in renal allograft outcome remains unclear. We conducted this study to determine whether posttransplant anti-HLA plays an important role in the outcome of renal allografts. Our investigation used a cross sectional design. Class I and II anti-HLA antibodies were obtained in 41 renal transplant patients. Patients had undergone either living-related (n = 15) or cadaveric (n = 26) RT. All patients had been transplanted for >6 months. The correlation of posttransplant class I and class II, anti-HLA antibodies with renal allograft function glomerular filtration rate (GFR) was analyzed. Patients displaying a GFR of >60 mL/min showed positive anti-HLA antibody status for class I (n = 2) and class II (n = 9). In contrast, those whose renal transplants showed a GFR <60 mL/min included three patients positive for HLA class I and 19 patients for HLA class II. Posttransplant class II anti-HLA antibody showed a negative correlation with GFR (r = -0.31, P = .03). Preliminary results indicated that class II posttransplant anti-HLA antibody might be one mechanism of chronic renal allograft rejection and may confirm the important role of HLA matching in renal transplantation outcome.     

Anti-idiotypic antibodies from highly sensitized patients stimulate B cells to produce anti-HLA antibodies

BACKGROUND: Sustained allosensitization increases waiting time for transplantation and increases the risk of rejection. The purpose of this study is to examine the effect of anti-idiotypic antibodies on B-cell responses and to define their role in alloantibody production. METHODS: The Immunoglobulin G (IgG) fraction, or the sera of 19 highly sensitized (HS) patients was absorbed to remove anticlass I antibody and was incubated with B cells. The culture supernatant was assayed for antihistocompatibility leukocyte antigen (HLA) antibody and tested for reactivity against a panel of normal lymphocytes. Similar studies were performed in 5 of the 19 patients who had a fall in alloantibody levels. RESULTS: The IgG (HS) fraction induced anti-HLA antibody from normal and autologous B cells in all 19 HS patients studied. The reactivity to HLA antigens in the culture supernatant was similar to the sera for each patient studied. The in vitro generated anti-HLA antibody bound to the IgG fraction used to stimulate the B cells. The in vitro production of anti-HLA antibodies was absent in the serum of all five patients who became nonsensitized. CONCLUSIONS: All patients who have high levels of alloantibody have anti-idiotypic antibodies in their sera that stimulate B cells to produce anti-HLA class I antibody similar in reactivity to that of their own sera. In the patients who have nondetectable alloantibodies in their sera, the stimulating anti-idiotypes are not measurable. Anti-idiotypic antibodies may act as a vaccine and cause sustained levels of alloantibody production.     

HLA class I antibody mediated accommodation of endothelial cells via the activation of PI3K/cAMP dependent PKA pathway

Allografts transplanted across ABO incompatibility or human leucocyte antigen (HLA)-sensitization undergoes antibody (Ab) mediated hyperacute rejection. Depleting anti-graft Ab from the recipient by plasmapheresis prior to transplantation can prevent this Ab-mediated rejection. Under these conditions, allografts have been shown to function even when the Ab rebound in the recipients. We have developed an in vitro model using human aortic endothelial cells (EC) and elucidated the ability of W6/32 HLA class I monoclonal Ab to provide signals following binding to MHC class I molecules. Using this model, we show that ECs undergo caspase 3-dependent cell death by apoptosis upon exposure to saturating concentrations of W6/32 and complement. In contrast, exposure of ECs to sub-saturating concentrations of W6/32 conferred resistance towards Ab/complement-mediated lysis that has been termed accommodation. Accommodated ECs exhibited a significant increase in the expression of anti-apoptotic genes Bcl-xL, Bcl-2 and Heme Oxygenase-1 and the induction of Phosphatidylinositol 3 kinase (PI3K) and cyclic adenosine monophosphate (cAMP) dependent protein kinase A activities that facilitate the phosphorylation of Bad at positions Ser(136) and Ser(112). In conclusion, exposure of sub-saturating concentrations of HLA class I Ab results in the induction of signals downstream that confers resistance to endothelial cells against Ab-complement mediated cell death. Together, the observations made in this study will provide the basis for delineating the molecular mechanisms involved in mediating accommodation and developing strategies to induce accommodation in grafts prior to transplantation in highly sensitized patients.     

Anti-CD20 antibody suppresses anti-HLA antibody formation in a HLA-A2 transgenic mouse model of sensitization

BackgroundB cell depletion by anti-CD20 antibody is used in desensitization protocols and for treatment of antibody-mediated rejection (AMR). However, little is known about the efficacy and the mechanism(s) of action.MethodsA mouse model of HLA sensitization was used to study the effectiveness of anti-CD20 treatment on B cell depletion and anti-HLA antibody suppression.ResultsImmunization of C57BL/6 mice with skin grafts from a transgenic C57BL.Tg/HLA-A2.1 mouse resulted in robust production of anti-HLA IgM and IgG antibodies, and accelerated rejection of a secondary skin allograft (within 3 days) featured by intragraft IgG and C4d deposition. Both IgM and IgG alloantibodies are specific to HLA-A2 as well as to a panel of class I HLA, including A1, A3, A25, A26, A29, and A30. These alloantibodies were complement-dependently cytotoxic (CDC) against HLA-A2 expressing target cells. Administration of 2 doses of a mouse–anti-mouse CD20 monoclonal antibody significantly reduced the levels of anti-HLA IgG2a antibodies, suppressed serum CDC, and prolonged survival of the secondary skin allografts. Suppression of anti-HLA IgG antibodies was associated with significant depletion of B220⁺/CD5⁻ B cells from the blood, the spleen and the bone marrow of the treated animals.ConclusionAnti-CD20 treatment effectively depletes B220⁺/CD5⁻ B cells, resulting in potent suppression of anti-HLA IgG and prolongation of skin graft survival. The data are in support for the use of anti-CD20 antibodies in highly-HLA sensitized patients undergoing desensitization and for the treatment of AMR.     

Hyperacute renal allograft rejection from anti-HLA class 1 antibody to B cells -antibody detection by two color FCXM was possible only after using pronase-digested donor lymphocytes

Abstract We present a report of a transplant recipient who lost her renal allograft from hyperacute rejection. This was secondary to a weak IgG anti-HLA class I antibody that was only reactive to donor B lymphocytes. This antibody was not detected in her pretransplant serum by the conventional complement-dependent cytotoxicity assays using donor blood lymphocytes. Pretransplant sera were analyzed retrospectively by two-color flow cytometric crossmatching (FCXM). It was difficult to determine if the recipient's serum contained an IgG antibody specific for HLA on donor B cells since IgG from control AB sera and pretransplant sera bound equally well to CD19 B cells. However, when donor lymphocytes were pretreated with pronase to digest the membrane receptor for Fc domain of IgG (FcyR) on non-T-cells, control IgG in AB serum did not bind to B cells and, hence, it was easy to detect binding of IgG (in pretransplant sera) to HLA on B cells. This case underscores the importance of identifying weak anti-HLA class I antibodies reactive only to B cells. Moreover, it shows that the currently used two-color FCXM lacks the specificity to detect such antibodies.     

双滤过法血浆分离治疗移植肾术后排斥反应及术前高度致敏

目的探讨肾移植术后排斥反应和术前高度致敏的治疗方法。方法采用双滤过法血浆分离术（ＤＦＰＰ）治疗３６例术后排斥反应和９例术前高度致敏患者。结果２４例急性排斥中２２例逆转,１０例加速性排斥（ＡＣＲ）全部逆转,１例慢性排斥肾功能稳定,１例超急性排斥（ＨＡＲ）摘除移植肾;排斥反应采用ＤＦＰＰ治疗者其一年移植肾生存率为８４．０％。去致敏抗体者８例已接受肾移植,其中１例术后发生ＨＡＲ。结论认为ＤＦＰＰ是治疗排斥反应和预防高敏患者术后发生致敏抗体介导的ＨＡＲ和ＡＣＲ的有效方法,显著提高了排斥反应的逆转率和移植肾生存率     

Hyperacute renal allograft rejection from anti-HLA class 1 antibody to B cells —antibody detection by two color FCXM was possible only after using pronase-digested donor lymphocytes

We present a report of a transplant recipient who lost her renal allograft from hyperacute rejection. This was secondary to a weak IgG anti-HLA class I antibody that was only reactive to donor B lymphocytes. This antibody was not detected in her pretransplant serum by the conventional complement-dependent cytotoxicity assays using donor blood lymphocytes. Pretransplant sera were analyzed retrospectively by two-color flow cytometric crossmatching (FCXM). It was difficult to determine if the recipient's serum contained an IgG antibody specific for HLA on donor B cells since IgG from control AB sera and pretransplant sera bound equally well to CD19 B cells. However, when donor lymphocytes were pretreated with pronase to digest the membrane receptor for Fc domain of IgG (FcγR) on non-T-cells, control IgG in AB serum did not bind to B cells and, hence, it was easy to detect binding of IgG (in pretransplant sera) to HLA on B cells. This case underscores the importance of identifying weak anti-HLA class I antibodies reactive only to B cells. Moreover, it shows that the currently used two-color FCXM lacks the specificity to detect such antibodies.