Middle East respiratory syndrome coronavirus (MERS-CoV) continues to cause outbreaks in humans as a result of spillover events from dromedaries. In contrast to humans, MERS-CoV–exposed dromedaries develop only very mild infections and exceptionally potent virus-neutralizing antibody responses. These strong antibody responses may be caused by affinity maturation as a result of repeated exposure to the virus or by the fact that dromedaries—apart from conventional antibodies—have relatively unique, heavy chain–only antibodies (HCAbs). These HCAbs are devoid of light chains and have long complementarity-determining regions with unique epitope binding properties, allowing them to recognize and bind with high affinity to epitopes not recognized by conventional antibodies. Through direct cloning and expression of the variable heavy chains (VHHs) of HCAbs from the bone marrow of MERS-CoV–infected dromedaries, we identified several MERS-CoV–specific VHHs or nanobodies. In vitro, these VHHs efficiently blocked virus entry at picomolar concentrations. The selected VHHs bind with exceptionally high affinity to the receptor binding domain of the viral spike protein. Furthermore, camel/human chimeric HCAbs—composed of the camel VHH linked to a human Fc domain lacking the CH1 exon—had an extended half-life in the serum and protected mice against a lethal MERS-CoV challenge. HCAbs represent a promising alternative strategy to develop novel interventions not only for MERS-CoV but also for other emerging pathogens.
The availability of vaccines in response to newly emerging infections is impeded by the length of time it takes to design, manufacture, and evaluate vaccines for clinical use. Historically, the process of vaccine development through to licensure requires decades; however, clinicians and public health officials are often faced with outbreaks of viral diseases, sometimes of a pandemic nature that would require vaccines for adequate control. New viral diseases emerge from zoonotic and vectorborne sources, such as Middle East Respiratory Syndrome coronavirus and Chikungunya, and while these diseases are often detected in resource-rich countries, they usually begin in low- and mid-income countries.1 Therefore, part of the timeline for a vaccine involves surveillance and detection of new pathogens in remote areas and transfer of specimens to laboratories capable of vaccine development.
Rift Valley fever virus (RVFV) is a member of the family Bunyaviridae and can lead to severe diseases in humans and livestock. Although most human infections proceed as mild flu-like illness, severe manifestations as retinitis, meningoencephalitis or even hemorrhagic fever syndromes due to fulminant hepatitis do occur in about 1-2% of the cases. Infections of adult ruminants and camels rarely lead to manifest and lethal hepatitis, but are rather observed as febrile diseases. However, so called ‘abortion storms’ are characteristic for RVFV infections of pregnant ruminants, leading to an up to 100% mortality rate in new borne animals. While human infections are mostly caused by contact to viremic animals, the transmission through RVFV-infected mosquitoes is of major importance for livestock and wildlife. To date RVFV was found in more than 30 mosquito species. Currently RVFV is widely endemic in Africa, recurrently causing substantial outbreaks. Significant losses in human and animal populations highlight the major impact of the pathogen for both healthcare and animal husbandry. For mitigation and monitoring of these impacts, knowledge of the specific infection ecology is of particular importance. To address these issues, a cross-regional serological and molecular screening of livestock sera in Cameroon and Mauritania was implemented. The findings in Cameroon demonstrated considerable inter-species differences, reflected by a significantly higher seroprevalence of cattle compared to small ruminants. Additionally, striking regional variabilities of seropositivity were observed, implicating a decline from north to south Cameroon. Apart from general seroconversion, acute infections were detected for the first time in three cattle and one small ruminant, harboring RVFV-specific IgM antibodies. Moreover, virus derived RNA was detected in one IgM positive cattle, indicating the existence of low-level circulation of RVFV. By providing first evidence of acute infections, both the existence of an ongoing enzootic cycle and the potential for severe outbreaks in future was demonstrated. Although recurrent RVFV outbreaks in Mauritania led to massive losses in the past, serological and molecular investigations during inter-epidemic periods are absent to date. Therefore, samples of small ruminants, cattle and camels that were collected during inter-epidemic periods from 2012-2013, were analyzed. Comparative analyses demonstrated a significant difference in small ruminants, showing a strong decline of seroprevalence during inter-epidemic periods. In contrast, the rate of seropositivity in camels and cattle was almost identical to those detected during epidemics. Obtained data do therefore clarify the significant role of small ruminants as important sentinels for RVFV, as a remarkable increase of seroconversion will indicate a possible introduction of RVFV into the herds. Furthermore the evidence of an IgM positive cattle harboring viral RNA illustrated the presence of an enzootic cycle. Camelids play a yet neglected but pivotal role in transmission and spread of RVFV and associations with human infections highlight the eminent need for effective vaccines for this species. For this purpose alpacas were chosen as model organisms for camelids and were immunized with the live attenuated MP-12 vaccine, evaluating its safety, immunogenicity and pathogenicity. The application of MP-12 proved to be safe as no shedding of vaccine virus was recorded and no persisting alterations in hematology and clinical chemistry were observed. Additionally the vaccine was highly immunogenic, as stable neutralizing antibody titers were generated by a single application. A detailed investigation of antigen-specific reactivity demonstrated a significant generation of antibodies directed against NSs, NP and Gn proteins. A minimal residual pathogenicity was demonstrated in alpacas 3 dpi as a replicative potential was verified in serum and liver. In addition pathological examinations revealed a mild, multifocal, acute necrotizing hepatitis with antigenic presence of NP, Gn, Gc and NSm. In contrast, hepatic lesions 31 dpi displayed a lymphohistiocytic character, indicating the efficient immunological clearance and absence of sequelae. Furthermore, next generation sequencing of recovered MP-12 confirmed the genetic stability of the vaccine. Therefore MP-12 is a safe and immunogenic vaccine for camelids, yet with considerable residual pathogenicity. In summary, the here presented results elucidate characteristics of the RVFV infection ecology in Cameroon, present comparative analyses during inter-epidemic periods in Mauritania and evaluate the suitability of the RVFV vaccine MP-12 for camelids. The obtained data can be used for awareness raising and risk assessment of Rift Valley fever as well as for the development of prevention strategies.