The Lyme disease (LD) vaccine formerly
approved for use
in the
United States consisted of recombinant outer surface protein A (OspA)
from Borrelia burgdorferi sensu stricto
(ss), the bacterial genospecies responsible for the vast majority
of LD in North America. OspA is an ∼30 kDa lipoprotein made
up of 21 antiparallel β-strands and a C-terminal α-helix.
In clinical trials, protection against LD following vaccination correlated
with serum antibody titers against a single epitope near the C-terminus
of OspA, as defined by the mouse monoclonal antibody (MAb), LA-2.
However, the breadth of the human antibody response to OspA following
vaccination remains undefined even as next-generation multivalent
OspA-based vaccines are under development. In this report, we employed
hydrogen exchange–mass spectrometry (HX-MS) to localize the
epitopes recognized by a unique panel of OspA human MAbs, including
four shown to passively protect mice against experimental B. burgdorferi infection and one isolated from a
patient with antibiotic refractory Lyme arthritis. The epitopes grouped
into three spatially distinct bins that, together, encompass more
than half the surface-exposed area of OspA. The bins corresponded
to OspA β-strands 8–10 (bin 1), 11–13 (bin 2),
and 16–20 plus the C-terminal α-helix (bin 3). Bin 3
was further divided into sub-bins relative to LA-2’s epitope.
MAbs with complement-dependent borreliacidal activity, as well as B. burgdorferi transmission-blocking activity in
the mouse model were found within each bin. Therefore, the resulting
B cell epitope map encompasses functionally important targets on OspA
that likely contribute to immunity to B. burgdorferi.