Autoimmune anti-D in an RhD-positive pregnant woman: A case report

Background

The RhD antigen is highly immunogenic, with potential for inducing anti-D antibodies. The most common mechanism for forming anti-D antibodies is alloimmune, where an Rh negative woman is exposed to RhD positive blood. Alloimmune anti-D can cause severe haemolytic disease of the fetus and newborn (HDFN). ¹ Antenatal prophylaxis with RhD immunoglobulin (RhDIg) has substantially reduced rates of anti-D alloimmunisation and HDFN in RhD-negative women.^1–3 The other mechanism for forming anti-D antibodies is autoimmune. Autoimmune anti-D is a rare entity, associated with autoimmune haemolytic anaemia in case reports,^4–10 but with limited literature regarding its risk of HDFN. ¹¹ We present our approach to a case of autoimmune anti-D in an RhD-positive pregnant woman.

Case presentation

A 30-year-old woman, with a B positive blood group was found to have anti-D antibodies on antenatal testing at 10 weeks, gestation. She had two normal vaginal births and one miscarriage previously. She had never received RhDIg or transfusions. There was no history of autoimmune disease or haemolytic anaemia.

The Australian Red Cross Lifeblood Red Cell Reference Laboratory confirmed that the anti-D antibodies were IgG with potential to cross the placenta. This was confirmed by indirect antiglobulin testing, which involves incubating the patient's plasma with red blood cells of known antigen types, then adding an anti-human globulin reagent that causes agglutination if there is an IgG antibody corresponding to a red cell antigen. In this case, the patient plasma reacted only against D positive red cells (Table 1). The anti-D identified was thought to be autoimmune as reactions were also seen when patient plasma was tested against their own red cells (i.e. positive auto control). RHD genotyping further confirmed the patient's RhD positivity without D variants. The autoimmune anti-D showed high titre low avidity (HTLA) behaviour, as demonstrated by an anti-D titre of 1:128, weak reactions against RhD positive cells, and difficulty using the patient's red cells to completely adsorb anti-D reflecting low avidity binding.

OPEN IN VIEWER

Table 1.

Summary of serological reactions observed.

	Tube testing			Column agglutination technique
	RT	PEG IAT	PAP IAT	DTT IAT	IAT	Autoadsorbed plasma	Autoadsorbed eluate
R1R1 (CCDee)	0	w	tr	1+	1+	2+	3+
R2R2 (ccDEE)	0	1+	w	2+	2+	2+	3+
r'r (Ccdee)	0	0	0	0	0	0	0
r"r (ccdEe)	0	0	0	0	0	0	0
Rr (ccdee)	0	0	0	0	0	0	0
Auto control	0	w	0	1+	w	2+	3+

RT: room temperature; IAT: indirect antiglobulin test; PEG: polyethylene glycol; PAP: papain, DTT: dithiothreitol; tr: trace; w: weak.

Low avidity reactions (trace to 2+  on scale of 0–4; bold) are seen with D+  red cells using tube and column agglutination techniques. Reactions seen with the patient's own red cells (auto control) suggest an autoantibody. Reactions only occurred with IAT, indicating an IgG antibody. Papain and DTT excluded other antibodies. An autoadsorption using the patient's red cells to bind autoantibodies was only partially successful as the autoadsorbed plasma retained 2+  reactions against D+  red cells, reflecting low avidity binding. An elution (removal of bound antibodies for identification) performed on adsorbed red cells was positive for anti-D.

Given the potential for severe HDFN with alloimmune anti-D, and limited evidence on managing autoimmune anti-D, ¹¹ the pregnancy was treated as high risk with maternal fetal medicine specialist oversight. Anti-D titres were performed monthly until 28 weeks gestation, then fortnightly until delivery, remaining elevated up to 1:512. Anti-D quantitation remained within the moderate HDFN risk range per RCOG ¹² and ANZBT ¹³ guidelines at 11.3–14.4 IU/mL. Weekly fetal middle cerebral artery (MCA) Doppler assessments were normal throughout pregnancy. Non-invasive prenatal analysis (NIPA) of fetal RhD type is not possible in RhD-positive mothers or those with RhD variants, and invasive testing (e.g. amniocentesis) was not warranted given reassuring ultrasound monitoring. Maternal haemoglobin was optimised prenatally, and extended Rh phenotype matched red cells were recommended in the event of transfusions to avoid formation of additional red cell antibodies (R1r, otherwise known as CcDee, was recommended in this case). The neonate was not expected to require transfusion given reassuring monitoring, however if required, blood would need to be ABO compatible and extended Rh phenotype matched. The patient had an uncomplicated birth at 38 weeks and 3 days of gestation. The baby was well, with a normal haemoglobin 162 g/L and no jaundice. Cord blood samples determined the baby was B RhD positive with a positive direct antiglobulin test (DAT) and eluate positive for Anti-D. At six weeks postpartum, there was persistent maternal autoimmune anti-D.

Discussion

We describe a rare case of autoimmune anti-D in an RhD-positive pregnant woman without maternal haemolysis or HDFN. To our knowledge, there has only been one other case with a similarly benign course. ¹¹ Given its rarity, it should not be assumed all cases will have a benign outcome. A meta-analysis of 51 women with autoimmune haemolytic anaemia in pregnancy found 10% had stillbirths and 59% neonatal haemolysis. ¹⁴

In the more common case of alloimmune anti-D in pregnancy, typical management involves^12,13

Paternal RhD typing and/or NIPA to establish whether the fetus is RhD positive (i.e. whether the fetus has red cells that anti-D antibodies can bind to);

Unfortunately, when the mother is RhD positive, typical pathways are not helpful. Fetal RHD inheritance cannot be assessed through paternal typing because the fetus could inherit the RHD gene from the mother. RHD NIPA cannot be used to confirm fetal RHD type in RhD-positive mothers. ¹⁵ RHD NIPA uses cell-free fetal DNA fragments in the maternal circulation, looking for the presence of parts of the RHD gene, indicating an RhD-positive fetus. In RhD-positive mothers, the maternal DNA far exceeds fetal DNA in amount and obscures the fetal signal. Anti-D quantitation is similarly only validated for alloimmune anti-D.

Antibody behaviour on transfusion testing may help predict clinical outcomes. In our case, the autoimmune anti-D demonstrated HTLA properties. This refers to an antibody present in high amounts but with little ability to bind and cause haemolysis. This likely explained the lack of maternal haemolysis we observed and provided some reassurance that avidity to fetal red cells would be similarly low. Nevertheless, it is prudent in any case of autoimmune antibodies in pregnancy to monitor closely for maternal, fetal and neonatal haemolysis including testing cord blood at birth for haemoglobin, bilirubin and DAT.^16–18

RHD genotyping must be performed to exclude RhD variants (weak or altered RhD) when autoimmune anti-D is suspected, as there have been cases of erroneously assumed autoimmune anti-D resulting in HDFN.^16–18 Women with certain RhD variants may require RhDIg to avoid alloimmunisation.

Autoimmune anti-D also impacts the selection of appropriate blood products for transfusions. Ideally, this should be anticipated by the multidisciplinary team, and antenatal iron supplementation and prevention of postpartum haemorrhage should be used to limit the need for transfusion. If transfusion is required, the typical inclination would be to provide RhD-negative blood of rr phenotype (ccddee). However, this could alloimmunise against the c antigen which can cause significant HDFN in future pregnancies. Therefore, extended Rh-phenotyped blood matched to the mother should be provided.

Conclusion

Autoimmune anti-D is rare with limited literature about its management in pregnancy. Although not observed in our case, it has a theoretical risk of causing autoimmune haemolytic anaemia and HDFN. Successful management requires multidisciplinary collaboration between obstetrics and haematology. RHD genotyping is crucial to exclude RhD variants. Given limited evidence, close monitoring for maternal haemolytic anaemia and HDFN is warranted. If transfusions are required, Rh phenotype matched red cells are recommended to prevent alloimmunisation.