Tick-borne Encephalitis Virus Vaccines Contain Non-structural Protein 1 (NS1) Antigen and May Elicit NS1-Specific Antibody Responses in Vaccinated Individuals

Abstract

Vaccination against tick-borne encephalitis (TBE) is based on the use of formalin-inactivated, culture-derived whole-virus vaccines. Immune response following vaccination is primarily directed to the viral envelope (E) protein, the major viral surface antigen. In Europe, two TBE vaccines are available in adult and pediatric formulations, FSME-IMMUN® (Pfizer) and Encepur® (GlaxoSmithKline). Herein, we analyzed the content of these vaccines using mass spectrometry (MS). The MS analysis revealed that the Encepur vaccine contains not only proteins of the whole virus particle, but also viral non-structural protein 1 (NS1). MS analysis of the FSME-IMMUN vaccine failed due to the high content of human serum albumin used as a stabilizer in the vaccine. However, the presence of NS1 in FSME-IMMUN was confirmed by immunization of mice with six doses of this vaccine, which led to a robust anti-NS1 antibody response. NS1-specific western blot analysis detected anti-NS1 antibodies also in sera of humans who received multiple doses of either of these two vaccines; however, most vaccinees who received ≤3 doses were negative for NS1-specific antibodies. The contribution of NS1-specific antibodies to protection against TBE was demonstrated by immunization of mice with purified NS1 antigen, which led to a significant (p < 0.01) prolongation of the mean survival time after lethal virus challenge. This indicates that stimulation of anti-NS1 immunity by the TBE vaccines may increase their protective effect.

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“…Several studies showed that NS1 immunization or passive transfer of anti-NS1 antibodies afforded protection against infections with flaviviruses, such as DENV, YFV and WNV in animal models [ 117 , 118 , 119 , 120 , 121 , 122 , 123 ]. These findings have been confirmed for the TBEV NS1 protein [ 97 , 101 , 105 , 106 , 107 , 108 , 124 ]. Mouse studies showed that immunization with a synthetic peptide corresponding to the structurally conserved α-helix (aa37–55) of NS1 [ 108 ] or with various synthetic fragments of NS1 [ 101 ] induced partly protective immunity against lethal challenge infection with TBEV.…”

“…In its pentameric or hexameric form, NS1 is secreted into the extracellular space [ 11 ], inducing a NS1-specific antibody response. After natural TBEV infection, NS1-specific IgM and IgG antibodies are detectable at high levels in human sera ( Table 2 ) [ 94 , 95 , 96 , 97 ]. Several studies showed that NS1 immunization or passive transfer of anti-NS1 antibodies afforded protection against infections with flaviviruses, such as DENV, YFV and WNV in animal models [ 117 , 118 , 119 , 120 , 121 , 122 , 123 ].…”

“…Mouse studies showed that immunization with a synthetic peptide corresponding to the structurally conserved α-helix (aa37–55) of NS1 [ 108 ] or with various synthetic fragments of NS1 [ 101 ] induced partly protective immunity against lethal challenge infection with TBEV. In addition, significantly prolonged survival of TBEV-infected mice was observed after hyperimmunization with a whole recombinant TBEV NS1 protein, although all mice succumbed to infection [ 97 ].…”

“…However, recent studies on the European vaccines question these findings by demonstrating the presence of NS1 in Encepur by mass spectrometry and detection of NS1-specific antibodies by Western blot in sera of vaccinees who repeatedly received Encepur or FSME-IMMUN. Moreover, vaccination with FSME-IMMUN induced a robust anti-NS1 antibody response in mice after several immunizations [ 97 ]. Further investigations are needed to confirm these results and to gain a better understanding of the role of vaccine-induced anti-NS1 antibodies.…”

“…Besides VN antibodies against the E protein, anti-NS1 antibodies were detectable in sera of TBE patients [ 94 , 95 , 96 , 97 ]. Mouse studies showed that these antibodies afforded partial protection from TBEV infection [ 97 , 101 , 108 ]. Therefore, the NS1 protein may be considered a promising additional vaccine target in future TBEV vaccines.…”