Passive antibody immunization against prion protein prevents the spread of clinical disease to the brain of mice, even though the antibody works primarily by reducing prion replication in peripheral tissues, including the spleen. This is the astonishing news in a report in today’s Nature by Anthony White, John Collinge, Simon Hawke, and colleagues at Imperial College and University College, both in London, as well as David Anstee of the National Blood Service.

Prion diseases and Alzheimer’s share key features in that both involve natural proteins that somehow misfold, aggregate, and destroy brain cells. Consequently, researchers are pursuing some parallel treatment strategies to interfere with this process, immunotherapy being one of them. Today’s report takes into mice prior work showing that antibodies can eradicate prion infection in cultured cells (see ARF related news story).

In the present study, White and colleagues used a model that develops fatal clinical scrapie after inoculation with scrapie brain homogenate. Peripheral inoculation models an earlier infection stage and slower disease course; intracerebral inoculation models a more advanced stage and rapid course. The researchers treated these mice repeatedly with two monoclonal antibodies: ICSM 35 is an IgG2b isotype that has high affinity to both the normal cellular and the infectious forms of the offending protein, PrPc and PrPSc, respectively. ICSM 18 is a IgG1 with high affinity only for PrPc. Western blots, immunoblots, immunohistochemistry, and bioassays of splenic homogenates all yielded similar results, namely that both antibodies depleted PrPSc in the spleen in scrapie-infected mice, with ICSM 18 slightly outperforming 35.

PrPSc levels in the spleen decreased markedly, regardless of whether the mice had received the scrapie inoculation intraperitoneally or intracerebrally. But did that do the mice any good? It did not help those that had received intracerebral inoculation; these mice survived to around 150 days after inoculation, as did untreated controls. It also did nothing for mice that had been infected intraperitoneally but were already developing clinical scrapie by the time they received the antibody; these mice died at around 197 days after inoculation, as did untreated controls. By contrast, mice that received antibody up to a month after intraperitoneal scrapie inoculation-even those in whom splenic scrapie replication was occurring at full speed-all survived to more than 500 days at submission of the paper. At 500 days, the treated mice had not yet shown signs of clinical scrapie, and treated mice that were killed 230 days after intraperitoneal infection had no detectable PrPSc in their brains. The scientists are currently assessing whether the prion infection is suppressed or eradicated in these mice.

Previous therapeutic interventions work only when given before or right around the time of inoculation, probably because they simply neutralize the injected material, the authors write (e.g., Sigurdsson et al., 2002, but see also Sigurdsson et al., 2003). That the present treatment works later, at the height of splenic prion accumulation, suggests it truly suppresses prion replication. This raises hope that there might be a window of opportunity after infection but before clinical signs appear, where immunization might be effective. However, a diagnostic test capable of identifying those people who harbor early-stage prion infections would be required, as well. The antibodies’ lack of success after clinical onset of disease likely results from their inability to cross the blood-brain barrier, the authors note. Finally, they caution that autoimmune reactions, which are now under intense study for AD immunization after they derailed a phase II trial (see ARF related online discussion) could well develop in patients with Creutzfeldt-Jakob or other prion diseases.—Gabrielle Strobel

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  1. This is a very exciting development for a rapidly fatal disease, for which there is no known therapy. Particularly important is that the passive immunotherapy can be started relatively late in the replication phase of the disease. While not identical, both Alzheimer's and prion disorders result in accumulations of fibrils of conformationally abnormal proteins that cause neurodegeneration. If immunotherapy shows any benefit in human prion disorders, it should encourage further development of immunotherapy for Alzheimer's patients. To my knowledge, this is the only therapy which appears to "cure" scrapie in mice.

  2. We are pleased that White and colleagues confirm our recent findings that anti-prion antibodies have the potential to be used as prophylaxes following scrapie exposure (Sigurdsson et al., 2002; Sigurdsson et al., 2003). We were surprised that they did not quote our 2003 study that was published before their paper was accepted. In addition, the editors of Nature were well aware of our work, as we submitted it to their journal in June 2002. Together, these in-vivo studies support previous in-vitro findings and results from transgenic mice expressing anti-prion antibodies, as referenced in our articles.

    By administering 2 mg of anti-prion antibodies twice a week, White et al. achieved a substantially better therapeutic effect than we did by injecting 50 μg once a week. Although extrapolation of an effective dose in a mouse to a human dose is not an exact science, 2 mg/20 g mouse corresponds to a 6 g/60 kg individual. Hopefully, a clinically effective prophylactic dose in humans will be closer to the dose we administered.

    To avoid any misunderstanding, we would like to point out that in one of our effective treatment paradigms, we initiated active immunization 24 hours after scrapie infection (Sigurdsson et al., 2002). This would result in detectable anti-prion antibodies probably over a month following exposure. Although this rescue approach had a less dramatic effect than our prophylactic treatment, it cannot be interpreted as a simple neutralization of the inoculum, as stated in the White et al. paper. Rather, it indicates that the antibodies may in some way be interfering with PrPSc-mediated conversion of PrPC to PrPSc, and/or increasing clearance of endogenous PrPSc.

    References:

    . Immunization delays the onset of prion disease in mice. Am J Pathol. 2002 Jul;161(1):13-7. PubMed.

    . Anti-prion antibodies for prophylaxis following prion exposure in mice. Neurosci Lett. 2003 Jan 23;336(3):185-7. PubMed.

  3. Having proof that established prion replication in the living situation can be controlled, there is no reason why these mouse monoclonal antibodies should not be humanized and infused into the brains of patients with human prion diseases.

References

News Citations

  1. Antibody Clears Prion Infection

Paper Citations

  1. . Immunization delays the onset of prion disease in mice. Am J Pathol. 2002 Jul;161(1):13-7. PubMed.
  2. . Anti-prion antibodies for prophylaxis following prion exposure in mice. Neurosci Lett. 2003 Jan 23;336(3):185-7. PubMed.

Other Citations

  1. ARF related online discussion

Further Reading

Primary Papers

  1. . Monoclonal antibodies inhibit prion replication and delay the development of prion disease. Nature. 2003 Mar 6;422(6927):80-3. PubMed.