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Breakthroughs lead to better understanding in prevention of transfusion-transmitted infection

09.12.2003


While the nation’s blood supply is safe overall and there is a relatively small likelihood that transfusion recipients will acquire a transmitted disease, there are still risks involved when transferring one person’s blood into another. The advent of new diseases, such as West Nile Virus (WNV), increases the need for further clinical vigilance and improved screening methods.



The blood supply is currently inspected in minipools with nucleic acid testing for HIV, hepatitis C virus (HCV) and WNV, but there is now a movement toward screening individual blood donations with such examinations. Additionally, researchers are learning more about reducing the risk of transferring specific diseases that may not affect healthy individuals but may seriously hurt immunocompromised patients, who often receive donated blood. Several studies of this nature are being presented during the 45th Annual Meeting of the American Society of Hematology.

"Modern medicine has contributed exponentially to the quality of medical care and has provided us with the opportunity to conduct successful blood transmissions to save millions of lives every year," said Stanley Schrier, M.D., President-Elect of the American Society of Hematology and Active Emeritus Professor of Medicine in the Division of Hematology at Stanford University School of Medicine. "However, it remains duly important that we not compromise our patients’ overall well-being while trying to improve their situation."


Yield of West Nile Virus RNA Screening of U.S. Blood Donors

West Nile Virus (WNV) has spread rapidly in the United States over the past four years. In 2002, hospitals documented the first cases of WNV linked to blood transfusions, leading to an intense effort to develop and implement a screening program. Researchers from Blood Systems Laboratories, Tempe, Ariz., have introduced nucleic acid amplification technology (NAT) screening, which has prevented a substantial number of transmitted West Nile Viruses in the past few months.

"By quickly introducing West Nile Virus NAT screening, we’ve identified a large number of WNV infected units and seen a major improvement in the overall yield and cost-effectiveness of NAT screening, justifying the substantial investment in implementation of molecular screening of the blood supply," said Michael Busch, M.D., Ph.D., from Blood Systems Research Institute, San Francisco, Calif., and lead author of the study.

From July 1 through October 31, 2003, Blood Systems Laboratory screened samples from 681,567 blood donations for WNV ribonucleic acid (RNA) using a 16-member mini-pool screening strategy. WNV RNA screening used a NAT system developed by Gen-Probe Corporation (San Diego, Calif.), which is based on probe-capture of WNV RNA, transcription-mediated amplification (TMA), and hybridization protection assay technology. The WNV TMA assay has an analytical sensitivity of 4.5 RNA copies per mL (50 percent limit of detection), with a sensitivity of approximately 72 copies per mL when applied to minipools of plasma from 16 donors (current pooled sample size used for WNV, HIV-1, and HCV NAT testing). Reactive minipools are resolved by retesting the 16 donations comprising the pool, and reactive individual donation samples are confirmed by repeat WNV TMA on an index or alternative sample, as well as a supplemental PCR assay and donor follow-up.

A total of 296 donations (0.043%) tested WNV reactive; 212 of these donors were confirmed as WNV infected (0.031%, 1:3,215) based on detection of WNV RNA by PCR (polymerase chain reaction, an alternative NAT method which rapidly synthesizes large quantities of a given DNA segment) and/or WNV IgM antibody (high molecular weight immunoglobulins, including the primary antibodies released into blood early in the immune response) of donation and follow-up donor specimens. During late July and early August, rates of reactivity as high as 1:167, 1:226, 1:255, and 1:272 blood donors were observed in South Dakota, North Dakota, Colorado, and Texas, respectively. To determine if donations with low-level viremia were missed by minipool NAT, 11,240 donations from the high minipool NAT yield regions were retested by individual sample WNV TMA, and 14 additional confirmed infected units were identified. One of these low-level viremic units was determined to have infected a recipient who developed WNV minengoencephalitis.

In conclusion, NAT screening of blood donors for WNV has detected a substantial rate (1:3,215) of viremic donations (virus present in the blood of the host), and prevented a large number of recipient infections. Since approximately 25 percent of WNV infections in transfusion recipients result in significant morbidity, screening has prevented serious WNV disease. In contrast, NAT yield for HIV-1 (1:3.7 million) and hepatitis C virus (HCV, 1:266,000) results in prevention of approximately three HIV-1 and 50 HCV viremic seronegative donations per year.

Transfusion-Transmitted Cytomegalovirus (CMV) Infection: Elucidation of Role and Dose of Monocytes in a Murine Model

Transfusion-transmitted cytomegalovirus (TT-CMV) infections occur in up to three percent of immunocompromised patients, despite the use of CMV seronegative (antibody negative) and filtered blood components. Cytomegalovirus causes enlargement of cells in various organs and development of characteristic inclusions in the cytoplasm or nucleus, and can lead to abortion, stillbirth, or congenital defects if infecting an embryo in utero, and retinitis, pneumonia, and multi-organ failure in compromised patients. These infections can be a significant cause of morbidity and mortality, but efforts to further reduce the incidence of TT-CMV are hindered by limited understanding of the processes involved. To help define the cell qualities and number needed to cause the infection, researchers at Emory University developed a mouse model of TT-CMV using murine CMV (MCMV).

Cohorts of recipient mice were given varying doses of either monocyte-enriched (n=20) or granulocyte-enriched (n=20) leukocytes (white blood cells). Monocytes are mononuclear white blood cells, which can ingest dead or damaged cells and help defend against infection; granulocytes are white blood cells with granules of toxic chemicals, which enable them to digest foreign cells. The mice were euthanized at day 14 and analyzed for the presence of MCMV by PCR (polymerase chain reaction, which rapidly synthesizes large quantities of DNA segments). Negative controls included recipient mice injected with the CD11b-negative fraction; positive controls were those mice injected with the CD11b-positive, unsorted fraction.

In those mice receiving the granulocyte-enriched fraction, regardless of the dosage, none were PCR positive for MCMV, but in the mice receiving the monocyte-enriched fraction, the percent that were PCR positive for MCMV reduced as the dose decreased. Results show that within the conditions of the study, PCR positive higher doses of monocytes are capable of causing MCMV infection in mice, while PCR positive granulocytes are not capable of initiating the viral infection.

"This suggests that monocytes are likely to be more biologically significant in the transfusion-transmission of CMV, and are critical when considering the composition of leukoreduced blood components, especially the type and number of residual white blood cells," said Christopher Hillyer, M.D., of Emory University, Atlanta, and lead author of the study. "These data suggest that a minimum number of monocytes is required to induce TT-CMV."

In humans, this dose equals approximately 106 monocytes, a number well above the number of residual monocytes (approximately 50,000) following leukoreduction. Leukoreduced blood components are those filtered to remove white blood cells, which can carry the virus and thus cause disease.

Donor mice (n=100) were infected with MCMV (1 x 106 pfu Smith strain) and rested for 14 days to prepare murine leukocytes with latent CMV. Harvested white blood cells (WBCs) were washed to remove serum and any free virus, which were PCR positive and RT-PCR negative for MCMV (defining latency). The donor WBCs were subjected to positive selection using CD11b to obtain a monocyte/granulocyte fraction, which was also PCR positive for MCMV; the CD11b-negative fraction was PCR negative. The CD11b-positive monocyte/ granulocyte fraction was separated into monocyte-enriched (>95 percent by microscopy) and granulocyte-enriched fractions; both fractions were PCR positive for MCMV.


The American Society of Hematology is the world’s largest professional society concerned with the causes and treatment of blood disorders. Its mission is to further the understanding, diagnosis, treatment, and prevention of disorders affecting blood, bone marrow, and the immunologic, hemostatic, and vascular systems, by promoting research, clinical care, education, training, and advocacy in hematology.

Aimee Frank | EurekAlert!
Further information:
http://www.hematology.org/

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