HIV-leishmaniasis co-infection poses a major threat in people living with HIV (PLWHV) and vice versa [1]. HIV and leishmaniasis are restricted within the confined of mutual reinforcement [2]. In PLWHV, Leishmaniasis induces, hastens AIDS onset and limits lifespan [3]. Furthermore, HIV infection escalates the risk of clinical Visceral Leishmaniasis (a severe form of leishmaniasis) more than 100 times [4]. Both infections co-existence HIV-leishmaniasis co-infection increase disease burden by inhibiting T helper cell-mediated immune responses [1,4]. In addition, there is a fundamental change in cytokine network signaling pattern [1,4].

Leishmaniasis could be cutaneous (CL), mucocutaneous (ML) or Visceral (VL) depending on the etiologic species and the site or tissue/ organ affected. CL pose enormous health challenge in most global regions especially the third world countries [5]. At least 88 countries are considered endemic for leishmaniasis [6]. In sub-Saharan Africa, PLWHV are afflicted more frequently with VL as the third most common infection [7]. As a result of the VL affecting the destitute and indigent patients, under-reporting of the presented cases in several affected regions is usually due to the absence of diagnostic equipment to investigate the co-infection and to inefficient reporting systems [1,7].

Importantly, Individuals living with VL-HIV co-infection are regarded as super-spreaders of VL infection which presents a major challenge to the efforts of eradicating leishmaniasis [1]. Based on these reasons, it was proposed that Leishmania organisms are critical in the pathogenesis of HIV-1 infection. In addition, it was observed that L. donovani amastigotes enhances HIV replication via TNF-alpha production and activates CD4+ T cells with the aid of its lipophosphoglycan [8]. Put together, it has been proven that anti-leishmanial therapy is efficacious in the inhibition of HIV replication [9]. However, the balances of pro- and anti-inflammatory cytokines are needed to prevent immunopathological disorders [9].

Pro-inflammatory cytokines secreted by Th1 cells are instrumental in regulating the proliferation and differentiation of T-lymphocytes in defense against HIV-leishmania co-infection. Despite the role of these cytokines, it is vital that these population is balance with their anti-inflammatory counterparts so as to avoid unpleasant immunopathological defects.

These defects which could be due to unregulated Th1 cell activity and excessive role of pro-inflammatory cytokines can have negative impact on the survival of T cell clones. The T cells that survive are dysfunctional and lack the ability to increase in number as well as secrete specific interferon-gamma targeted at antigenic stimuli. Previous research has shown that CD4 T lymphocytes with diverse lymphokine patterns are more efficient compared to humoral B cells in reducing the burden of leishmania infection. On the other hand, the anti-inflammatory cytokines (IL-4, IL-10) when secreted by Th2 cells in discriminate proportions can promote the survival of parasites within cells and increase disease burden [1,10].

A scientific study currently reported the existence of a dominant cytokine profile secreted by Th1 cells and possess high protective role for pathogens compared to Th2 cytokines in HIV infection. It has been hypothesized that a shift from Th1 to Th2 cytokines depletes CD4 population and enhances viral replication in AIDS [9,11]. In order to determine the expression profile of Th1 and Th2 cytokines in the case of HIV-Leishmaniasis co-infection, this study aims to investigate the plasma levels of both pro- (IL-2, interferon-gamma) and anti-(IL-10) inflammatory cytokines as well as white cell differentials of persons coinfected by the HIV and leishmaniasis University of Abuja Teaching Hospital, Northcentral Nigeria.

Leishmania/HIV’s capability to influence host cellular immunity and co-exist in the lymphoid tissues indicates their ability to mutually communicate by mutually re-enforcing their replication when co-existing in the same host cells [11]. To the best of our knowledge, this is one of a few studies that demonstrates for the first time the influence of the interaction of these pathogens on cytokines and leukocytes which are orchestrators of the host defense system and play vital functions in modulating the host immune response against Leishmania/HIV co-infection. Comparative analysis of leukocyte and cytokine profiles from co- and mono-infected participants highlighted significant variations in immune response mounted against co-infection, confirming the ability of Leishmania and HIV to mutually interact at the immunological level.

Participants co-infected with Leishmania and HIV responded with an overall decrease in pro-inflammatory cytokine (IL-2, TNF-alpha) and the increase in anti-inflammatory cytokine (IL-10) release. Low level of these pro-inflammatory cytokines have been reported in Indian patients with active Leishmaniasis and elevated levels observed in those treated of Leishmaniasis [12]. IL-2 and TNF-alpha have been implicated in protective immunity towards parasitic and viral infections [13]. TNF-alpha, a vital mediator of both innate and adaptive inflammatory responses [14], is observed to possess a crucial role in the formation and maintenance of granuloma against visceral leishmania. This anti-parasitic function of TNF-alpha is mediated by the activation of infected macrophages for the killing of intracellular Leishmania amastigote [15]. IL-2, which is produced by activated CD4+ Th cells and was formally referred to as “T-cell growth factor”, have been thought to induce the proliferation and differentiation of Th2 cells (B-cells, natural killer cells, monocyte/macrophages, oligodendrocytes and lymphocyte activated killer cells) and aid in the release of IgG1 and IgE-producing cells which is vital for the resolution Leishmaniasis [16], suggesting that this cytokine may be crucial for Leishmania clearance. Hence decreased concentrations of IL-2 observed in the co-infected versus the mono-infected cohort, apart from the already diminished levels of TNF-alpha, may be indicative of a non-favourable, poor prognosis for Leishmania.

However, an increase in the level of IL-10 (an anti-inflammatory cytokine), and reduction in cell-mediated immunity are seen in Leishmania infection. IL-10 dampens the production of many pro-inflammatory cytokines (including TNF-alpha, IL-1, IL-6, and IL-12), it also decreases the expression of MHC-II and other co-stimulatory molecules on the surface of macrophages, and these inhibit macrophage-mediated activation of CD4+ helper T cells leading to a reduction in both adaptive and innate immune responses [17].

The immunosuppressive role of IL-10 in human visceral leishmaniasis leads to a drastic decrease in the accumulation of monocyte-derived macrophages, which is controlled by the migration inhibition factor. Furthermore, IL-10 is observed to encourage the Th1 cell dysfunction which enhance intracellular infection for interferon-gamma production, deactivating parasitized tissue macrophages, and downregulating antigen presentation by dendritic cells [17].

IL-10 have also been implicated in inhibiting the leishmanicidal roles of macrophages by decreasing the synthesis and release of pro-inflammatory molecules (reactive nitrogen intermediates by macrophages, interferon-gamma by T/natural killer cells, and IL-12 mediated activation of macrophages). These suppressive roles of IL-10 which is released by alternatively activated macrophages and interferon-gamma co-producing CD4+ T cells (type 1 regulatory T cells) at moderate to high levels due to the low to zero levels of pro-inflammatory cytokines and the decreased number of multifunctional CD4+ T cells [17]. During the initial phases of infection justifies why it is associated with decreased immunity against Leishmania infection. Due to suppressive role of IL-10, the parasite burden continues to build up even when interferon-gamma levels are elevated [18].

In addition to the varying levels of cytokine profile, this study revealed a higher secretion of IL-10 which was associated with increased viremia and depleted CD4+ T cell population in Leishmania/HIV co-infected participants compared to those of the HIV mono-infected group. This corroborated with a recent study, which reported CD4+ T cell count as low as 49 cells/mm³ in HIV/Leishmaniasis asymptomatic immune responders compared to the non-responders [19]. The marked fall in CD4+ T cell number is associated with the reduced leishmanicidal capacity of macrophages, and the replication and uncontrolled systemic spread of the parasite throughout the body [20] which increases the risk of progression to visceral leishmaniasis by 100 to 2320 times based on the mutual interaction between HIV and Leishmania [21].

Based on the leukocyte profile, HIV/Leishmania co-infected participants had significantly lower lymphocyte and neutrophil counts with significantly higher monocyte, basophil and eosinophil counts as observed in this study. These observations were in line with a study conducted in Brazil [22] which demonstrated significantly lower levels of lymphocyte and haemoglobin counts at diagnosis, and considerably higher levels of eosinophil count in the initial hematologic evaluation in over 50% of co-infected participants. Similar observations were observed in the liver of genetically susceptible mice infected with experimental visceral leishmaniasis [19].

The primary cells of mice that are infected by Leishmania donovani amastigotes are the liver resident tissue macrophages (Kupffer cells). These cells are observed to produce cytokine sand chemokines which recruit monocytes and neutrophils to the infection site during the initial days of infection, thus, further amplifying the production of more chemokines [23]. Neutrophils (primary antimicrobial cells) control infections by phagocytising and killing invading pathogens. While some of these pathogens are killed by the lysosomal effect of the neutrophil, others which are either obligate or facultative intracellular organism become resistant to the lysosomal enzymes and replicate in these cells which become apoptotic. These apoptotic neutrophils (without parasiticidal effect) either control Leishmania growth by undergoing depletion to reduce the load of this parasite and delaying the onset of leishmaniasis [24] or act as Trojan horses with delayed apoptosis by the parasite for onward transmission, infection and replications of these parasites in monocytes and macrophages that phagocytize the infected apoptotic neutrophils.

Another exciting outcome observed in this study is the decrease in neutrophil count. Leishmania infection interferes with several signaling pathways in immune cells [25]. Monocytes have been observed to possess antiparasitic function based on their plasticity and ability to differentiate into potent antigen-presenting or regulatory cells [26]. The recruitment of neutrophils, monocytes, and then the T cells into the Kupffer cells is crucial for the formation of granuloma around these cells which is required for controlling the growth of Leishmania/HIV co-infection [27].

The cellular immune-mediated interactions between Leishmania spp and HIV seem to determine the extent of immunological response that occur in the co-infected biological system as well as the intensity of the onset of the accompanying infection. Similar circumstances have been observed with other Leishmania co-infections, revealing that the inherent consequences that emerge from several pathogen-host relations need to resolve when designing Leishmania vaccine trials. Proper consideration of parasite interplays should be adopted when determining the best strategy for the treatment of Leishmania-HIV co-infections to achieve immune stability without inflicting harm during the clinical course of the infected individual.

Authors appreciate Head of Immunology Laboratory of University of Abuja Teaching Hospital for Logistic and technical support.