Data shown represent the first large-scale population study of Hpr levels in humans. Previous laboratory investigations, using a small number of samples, reported lower Hpr levels in the range of 0.02–0.05 mg/ml 
. This was similar to median levels found in our Caucasian controls (0.049 mg/mL) but significantly lower than those of the Gabonese children (0.28 mg/ml) described in this study. Several explanations can be put forward for these higher levels in Gabonese children. These include host genetics and prevalence of an APR. It is possible that an increase in gene copy number may contribute to higher plasma levels of Hpr in Gabonese compared to Caucasian children. Up to 6 copies of the Hpr gene have been found in the chromosomes of African-Americans, whereas only one copy was found in Caucasian-Americans 
. The low frequency of increased Hpr gene copy number in African-Americans genotyped (28%) suggests that it is unlikely to account for the relatively high proportion of children with levels of Hpr>0.1 mg/ml (91%) found in this Gabonese study population. The high prevalence of an APR due to malaria infection seems more likely to be the cause of elevated median concentrations, although considerable individual variation in levels was observed, as discussed below.
Hpr levels were found to decrease with age in the Gabonese children. This may be dependent upon intrinsic factors related to the age of the host. It is interesting to note that some acute phase proteins show age-related profiles, e.g. levels of Hp decrease with age, whereas others are unrelated to age, e.g. CRP 
. Levels of Hpr were higher in females than in males. This finding may be related to metabolism of HDL. It has been shown in a cohort of 10–15 year old males that testosterone decreased levels of HDL-cholesterol and apolipoprotein A-I, whereas estradiol increased levels of HDL-cholesterol 
. Hpr is found circulating in HDL and thus changes in HDL levels will result in changes in Hpr levels. Furthermore, there is a marked decrease in plasma levels of HDL during infection and inflammation 
Univariate analysis showed that Hpr was correlated with the acute phase protein Hp, but not CRP or albumin in Gabonese children. Hp levels are determined by several factors, including IL-6-dependent APR, Hp genotype, parasite density and hemolysis 
. This correlation between Hpr and Hp would suggest that the production and/or clearance of both these molecules may be under the influence of the same mechanisms. The positive correlations in Hpr and Hp levels in Gabonese children, the majority of whom are undergoing an APR, leads us to suggest that Hpr may also be an APP. Whilst longitudinal analyses within individuals will be required to prove that this is the case, it is plausible that during an APR both Hp and Hpr may be upregulated simultaneously as IL-6 responsive elements (IL-6RE) have been identified in both the Hp and Hpr promoters 
. Indeed if Hpr is an acute phase protein, it is a moderate one, i.e. a protein showing a 1- to 10-fold increase during an APR as there was a positive correlation with Hp but no direct correlation between Hpr and CRP levels 
. This is not unexpected; although the CRP gene also contains IL-6RE 
,the relative kinetics of acute phase proteins differ during an APR, IL-6 and IL-1β acting synergistically to induce promotion of CRP production 
. Thus CRP levels rapidly increase several hundred-fold and fall rapidly whereas Hp levels rise 2-4-fold and persist 
. Albumin is a negative acute phase protein, i.e. production is reduced in an APR 
Hpr levels were inversely correlated with parasitemia. Hp levels also decrease with increasing parasite density due to intravascular hemolysis and subsequent clearance of Hb-Hp complexes 
. Thus, levels of Hp and Hpr may be correlated due to similar clearance patterns. In support of this possible conclusion, we have also shown that Hpr correlates with Hp in a malaria endemic region of Papua New Guinea, in the absence of an APR (Imrie H, unpublished observations). Whilst Hpr is able to bind Hb, in contrast to Hp-Hb, Hpr-Hb does not promote any high-affinity binding to the scavenger receptor CD163, which clears Hp-Hb complexes from the circulation 
. Nevertheless, the mechanism of clearance of Hpr-Hb may be related to the clearance of Hp-Hb complexes. It is usual for some plasma Hp (0.9%) to be associated with HDL particles 
and thus we speculate that when HDL associated Hp binds to Hb, some HDL containing Hpr may be cleared concurrently. Alternatively other scavenger receptors may also exist which bind to both Hp-Hb and Hpr-Hb.
Hpr levels were also associated with Hp genotype; Hp2-2 individuals had significantly lower levels of Hpr compared to Hp1-1 and Hp 2-1 individuals. This may be related to the differential affinity of Hp genotypes for binding Hb and subsequent patterns of clearance. Whilst the affinity of the higher molecular weight polymers for Hb is generally lower than that of Hp1-1, their plasma levels are lower, reflecting the clearance of a greater molecular weight per molecule. Thus levels are in the phenotype order: Hp1-1>Hp2-1>Hp2-2 
. Hpr concentration was also related to Hp subtype, with Hp1S-1S having lower levels of Hpr compared to 1F-1F. Functional differences between the Hp sub-phenotypes are unknown and we have reported that subtype (1S or 1F) does not affect plasma concentration of Hp in these children 
Our results show that levels of Hpr vary between and within populations from the United Kingdom and Gabon. Since all humans are refractory to infection with T. b. brucei
, there can be no trypanolytic advantage in having higher levels in endemic areas. The closely-related Hp is a multifunctional protein (e.g. antibody-like properties, immunomodulation, iron metabolism) 
.We speculate that Hpr likewise has more than one role, possibly in innate resistance to other pathogens. Hp levels modulate TLF activity: low Hp levels (due to acute intravascular hemolysis) result in increased TLF activity in vitro
(10–40 fold), whereas, high levels of Hp suppress both the lytic capacity of TLF in vitro
and the ability to clear trypanosomes in vivo
in a mouse model 
. Thus low Hp levels, as occur in malaria, might also increase other activities of Hpr. We speculate that Hpr may have a role in resistance to malaria. It has been suggested that the binding of Hp-Hb to CD163 elicits IL-6 and IL-10 secretion. It may be that Hpr-Hb complexes also have immunomodulatory effects.
Our population-based observations indicate that a more dynamic view of the relative roles of Hpr and Hpr-Hb complexes needs to be considered in understanding innate immunity to African trypanosomes and possibly other pathogens that cause ahaptoglobinemia, such as the newly discovered Plasmodium
spp of humans and primates 
In vivo population data presented here show individual variation in Hpr and Hp levels caused by complex dynamics of Hpr and Hp levels in children living under the burden of malaria infection in African settings with consequences for the ability to control infection.