Contact Tracing. Part 2
The NP is the most abundant polypeptide in Lassa virions, with approximately 60 molecules per particle. Thus, detection of NP antigen may be the most sensitive diagnostic for LASV, particularly in samples presenting with low antigenemia. Thus LFI and NP Ag detection ELISA were developed into rapid and confirmatory diagnostic platforms, respectively. Additionally, sensitive assays for the detection of LASV-specific IgM and IgG (Corgenix Medical Corp.), based on recombinant LASV protein ELISA, were used to qualitatively measure immunoglobulin levels. Data from these sensitive assays suggest that G-1180 was naive to LASV exposure prior to this incident as he presented with very low NP-specific IgM levels on day 6, which progressively increased throughout the monitoring period. Conversely, GP1 and GP2-specific IgM titers were not statistically above background levels throughout the same period. On day 10, low IgG titers were detectable in G-1180 as determined by endpoint titers, which increased three-fold through day 14. Immunoglobulins were again measured on day 74 post onset of illness in G-1180, and surprisingly IgM titers against glycoprotein and NP had risen relative to day 14, rather than subsiding. An expected increase in IgG titers to LASV proteins was also observed. Immunoglobulin levels in G-1180 measured on day 108 post onset of illness revealed still elevated IgM titers as well as IgG titers against NP (data not shown). The IgM titer to NP dropped to approximately one third the qualitative level measured on day 74, whereas IgG rose slightly compared to the same time point. IgM and IgG levels against GP1 and GP2 were not significant on days 74 and 108. This phenomenon of persistently high IgM titers against LASV antigens for weeks, months or longer, in a significant proportion of convalescent patients is currently under investigation (Garry et al., unpublished data).
The most remarkable aspect of the cytokine profile obtained at the time of admission was the significant level of IL-12p70 measured in serum. This pleitropic proinflammatory cytokine is produced by phagocytic cells, B cells, and other antigen-presenting cells that modulate adaptive immune responses, promotes Th1 cell development and is a potent inducer of IFN-γ and other cytokines in peripheral blood T and NK cells. IFN-γ further enhances production of additional IL-12 and other pro-inflammatory cytokines by phagocytic cells. IL-12 induced IFN-γ acts in a positive feedback loop providing an amplification mechanism in the inflammatory response. Remarkably on day 7, IL-12p70 was not detected, but its decrease coincided with a rise in IFN-γ levels, which remained elevated through day 10, but were undetectable on day 11. Another increase in IL-12p70 levels was measured on days 11 and 12, the latter coinciding with a second spike in IFN-γ. Though IL-12p70 may have some impact on pathogenesis of Lassa fever, its role in arenaviral pathogenesis has not been described. Despite this profile early in the monitoring period other pro-inflammatory cytokines, such as IL-1β and TNF-α were undetectable, and IL-6 and IL-8 fluctuated in the lower end of the assay range. Elevated levels of IL-8 have been previously reported by Mahanty et al., 2001 to correlate with a positive outcome in acute Lassa fever infections, in addition to IFN-induced IP-10, which was not measured in these studies. A very notable albeit short increase in pro-inflammatory cytokines was measured on day 10, when IL-6 and IL-8 spiked to very high levels, in addition to a small spike in IL-1β and TNF-α. The increase of endogenous pyrogens IL-1β, TNF-α, IL-6, IL-8 on day 10 coincided with re-development of a mild fever, and significantly increased respiratory and pulse rates. These levels dropped to baseline or were undetectable 24 hours later. A single spike in IL-2, a cytokine important in the differentiation and survival of cytotoxic T cells, and a facilitator of immunoglobulins by B cells, was measured on day 11.