Isolation of tonsillar B cells.
We hypothesized that KSHV could establish latent infection in tonsillar B cells. To test this, we obtained deidentified tonsils from routine tonsillectomies and purified the B cell fraction by negative selection, thereby avoiding activation of surface receptors, which could potentially alter either KSHV tropism or target cell phenotype. Flow cytometry confirmed that the sorting procedure generated greater than 95% pure CD19+ B cells (data not shown). We then exposed the cells to fresh preparations of concentrated KSHV. In later experiments, to monitor potential infection of non-B cells as well as to assess potential differences in the pattern of B cell infection in a mixed culture, we omitted the physical B cell isolation step and infected tonsil mononuclear cells en masse. In this latter approach, we identified (and gated on) B cells within the cultures based on their expression of the Ig light chain κ or λ (see results below). We studied KSHV infection in tonsil cells from more than 50 donors in total, though the data in this study represent the outcome from 21 representative individuals.
Stable presence of viral genomes and pre-latent transcript expression in human tonsillar B cells following ex vivo inoculation with KSHV.
To detect potential KSHV infection in human tonsillar B cells ex vivo, we exposed the cells to purified KSHV, washed the cells extensively to remove unbound virus, and quantified the amount of viral genomic DNA that remained associated with the cells over time. At early time points after exposure, we found a relatively high number of KSHV genomes associated with the B cells. This level declined approximately 100-fold over 24–48 hours after infection (hpi) but then remained relatively stable for at least 5 days after infection (5 dpi) (Figure A).
Levels of viral genome and PAN transcript associated with human tonsillar B cells following ex vivo inoculation with KSHV.
We reasoned that the initially high levels of viral DNA may have represented bound but not internalized virus that eventually dissociated from the surface of the B cells. In support of this notion, we found that trypsinization at 2.5 hpi reduced the number of viral genomes by approximately 100-fold, similar to levels detected at 5 dpi without trypsinization (Figure A, compare right and left panels). This finding suggested that the viral DNA present at 5 dpi likely represented internalized KSHV genomic DNA, whereas the higher level of cell-associated virus at 2.5 hpi may have represented a transient binding of KSHV to the cell surface but not to actual entry receptors (32
). Consistent with this interpretation, we found that at 5 dpi, trypsin treatment of infected cells in a separate experiment had little effect on the levels of cell-associated KSHV DNA.
The persistence of a low number of KSHV genomes in the tonsillar B cell cultures was consistent with a latent, rather than lytic, infection. Although we could still detect low levels of infectious genomes in the supernatant 48 hpi, this likely represented residual virions from the initial inoculum, since inhibition of the viral polymerase by continuous treatment of the culture with 500 μM phosphonoacetic acid (PAA) (34
) did not decrease these levels (data not shown). Furthermore, lytic replication of even a small fraction of the cells would have resulted in increasing levels of intracellular viral DNA. Instead, the levels were essentially unchanged between 2 and 5 dpi (Figure A). Taken together, these data suggested that the initial decrease in cell-associated virus between 2.5 hpi and 48 hpi and the small amount of virus in the media 48 hpi represented dissociation of cell-bound virus from the initial inoculum, rather than de novo lytic replication. The long-term persistence of trypsin-resistant viral DNA in tonsillar B cells, however, indicated that, at a minimum, KSHV gained entry into the cells.
To test whether internalized KSHV genomes were transcriptionally active in tonsillar B cells, we quantified the levels of the viral polyadenylated nuclear (PAN
) RNA by quantitative RT-PCR (qRT-PCR) amplification of total RNA. PAN
RNA is a direct target of the KSHV transcriptional activator (RTA) and is highly abundant during lytic replication (35
). Endothelial cells express PAN
following de novo infection (36
), as do tonsillar cells from asymptomatic KSHV-infected patients (21
). In agreement with previous descriptions of this RNA as a component of the pre-latent (first termed “lytic burst”) transcription program following de novo KSHV infection of primary endothelial cells (36
), we detected PAN
in tonsillar B cells beginning at 2 hpi. The levels of PAN
peaked at 24 hpi but trended downward thereafter (Figure B). Of note, we detected PAN
in B cells from 14 of 14 tonsils that we assayed but found no evidence of this viral transcript in uninfected B cells or in no-RT control reactions.
On its own, PAN
expression, and especially increasing levels of PAN
expression, could be a bellwether for low levels of lytic replication. However, in light of (a) the transient nature of PAN
expression in our culture system, (b) the stable, rather than increasing, intracellular viral DNA levels after infection (Figure A), and (c) the lack of the PAA effect on background levels of viral DNA in the supernatant (discussed above), our detection of PAN
most likely reflected the transient burst of pre-latent genes that can occur after de novo infection (36
Expression and maintenance of LANA within human tonsillar B cells.
Because tonsillar B cells are a heterogeneous population comprising cells at various stages of cellular activation and differentiation, we hypothesized that infection would likely be unevenly distributed, potentially favoring a subset of susceptible cells that would be enriched for viral genomes. Furthermore, even among individual infected cells, we predicted that the number of viral genomes would vary stochastically, reflecting, at least initially, a Poisson distribution as we previously reported for de novo infection of other cell types (27
To determine whether KSHV could establish latent infection in human tonsillar B cells, as well as to determine which fraction of cells were susceptible to infection, we incubated KSHV-exposed B cells with a monoclonal antibody directed against LANA. LANA forms a series of dimers or higher-order multimers that tether the viral genome to the host cell chromatin during latency (29
). By immunofluorescence microscopy, episome-associated LANA appears as characteristic punctate nuclear dots, with increasing numbers of LANA dots correlating with increasing numbers of intracellular viral genomes (27
). We predicted that KSHV-infected B cells might be rare in our cultures, so we employed MIFC, a highly sensitive and specific high-throughput imaging technique, to identify and characterize KSHV-infected (LANA-positive) cells (39
). This approach combines the specificity of immunofluorescence microscopy with the statistical power of flow cytometry. To eliminate nonviable cells from our analyses (approximately 50% of tonsillar cells in our cultures die within 72 hours regardless of the presence of KSHV and in agreement with published findings; ref. 40
), we treated the cells with DNase I (41
) at the time of collection. We then confined our analysis to the cells that stained brightly for DAPI, which indicated that their cell membranes were impervious at the time of collection and had therefore excluded the DNase.
As we had predicted, MIFC analysis revealed that a small fraction of the total culture of tonsillar B cells expressed LANA in the characteristic punctate nuclear pattern 60–84 hours after exposure to KSHV (Figure A). We created an imaging algorithm within the MIFC analytical software to identify and enumerate cells that contained these nuclear LANA dots (see Methods). The percentage of tonsillar B cells that became LANA dot–positive following KSHV exposure varied by tonsil donor, ranging from 1% to 2%, but averaged approximately 1.5% (Figure B and additional results below). LANA dots were present in purified tonsillar B cells in a Poisson-like distribution as described previously (27
), with a mean of 1.7 dots/cell. We did not detect LANA dots in cells that had not been exposed to KSHV (Figure A) or were exposed to UV-inactivated KSHV (data not shown).
Expression and maintenance of LANA within human tonsillar B cells.
While we routinely detected LANA dots in primary B cells between 48 and 96 hpi and were able to consistently detect viral genomes up to 5 dpi, we wanted to confirm the stability of KSHV infection by following the presence of LANA dots in these cells over longer periods. However, as primary human B cells in standard lymphocyte media survive for only limited times in culture, we utilized a feeder layer of CD40L-expressing 3T3 fibroblast cells (CD40L-3T3 cells) to provide survival signals (42
) to our longer-term cultures. This system allowed us to follow KSHV-exposed tonsillar B cells for 1–2 weeks with a viability (trypan blue and DNase exclusion) of approximately 45%.
To assess whether the fraction of infected B cells was stable over time, we infected tonsil cells en masse and followed the B cells (identified by expression of Ig light chain) longitudinally for the presence of LANA. Figure C shows an example of such an experiment; LANA was not evident at 1.5 dpi, but rose to approximately 2% by 2.5 dpi. Furthermore, in a sister culture grown in the presence of CD40L-3T3 support cells, the B cells remained alive and infected for at least 1 week, and the fraction of LANA+ cells continued to increase. Figure C shows, with this specific tonsil, that the percentage of LANA+ cells rose to approximately 8% by 7 dpi. Although we did not definitively determine whether the increased level of infection within the culture at later time points was due to the enhanced survival or proliferation of infected cells or to infection of new targets, we did not detect significant lytic virion production in our system (discussed above), while we did document KSHV-driven proliferation (see below). We utilized the CD40L-3T3 feeder layer for this experiment as a means of maintaining cell viability while determining whether LANA dot–positive cells persisted at longer time points after infection. The results indicated that infection remained in the primary human tonsillar B cells for up to 2 weeks after infection.
We also found that long-term culture and/or the feeder cell coculture led to a slightly higher background LANA signal, with the automated software detecting nonspecific dots at 7 dpi more frequently than at 72 hpi (Figure C). Individual images of cells with aberrant LANA signal in the older cultures unexposed to KSHV appeared consistent with the nonspecific, usually atypical single-dot pattern found in unexposed cultures at 72 hpi (see Figure A, left, bottom row, for an example). This pattern of fluorescence may represent either nonspecific labeling with the monoclonal anti-LANA antibody or autofluorescence that is evident in older cultures in the same fluorescence channel as the Alexa Fluor 488–conjugated anti-LANA antibody. In marked contrast, the vast majority of the LANA+ cells in the KSHV-exposed cultures at all time points demonstrated a characteristic intranuclear punctate pattern, with many showing numerous distinct dots (compare background staining at 72 hpi in Figure A, bottom row on the left, with true LANA+ cells on the right). Within the confines of the time points we examined, these data indicate that KSHV can establish a latent-appearing (LANA+) infection in a small fraction of human tonsillar B cells.
KSHV targets λ B cells for stable LANA+ infection.
We consistently detected LANA dots in only a small fraction of the tonsillar B cells, supporting our hypothesis that latent (LANA+
) KSHV infection may be restricted to a subset of B cells. We further hypothesized, however, that this subset may resemble the KSHV-infected cells found in the KSHV-associated B cell tumor, MCD. MCD plasmablasts express exclusively the λ light chain of the B cell receptor, a phenotype that is fixed at the B cell precursor stage (43
). We hypothesized that the normal counterpart(s) of, or initial precursor to, KSHV tumor cells may also share this Ig light chain bias.
To identify the target B cell subset(s) as well as any potential non-B cell targets of LANA+
KSHV infection ex vivo, we exposed unpurified tonsil cell suspensions to KSHV and labeled these cells with antibodies directed against the Ig light chains κ and λ in addition to the viral protein LANA (Figure A) at 60–84 hpi. Consistent with earlier descriptions of tonsillar composition, we found that the tonsils we examined contained approximately 50%–70% B cells, 30%–50% T cells, and a small percentage of monocytes/dendritic cells (44
) both before KSHV exposure and 72 hpi (data not shown). Since expression of either κ or λ defines all B cells, and absence of both markers defines non-B cells, we employed this simple algorithm to define these two populations. Using MIFC analytical software, we gated the KSHV-exposed tonsil cells into non-B and B cells (30%–45% and 50%–65% of viable cells, respectively) (see Figure B for representative gating) and then assessed for the presence of LANA dots in each of the two populations. In all of our analyses, we similarly assessed for LANA+
cells in control cultures that were not exposed to KSHV to confirm the specificity of anti-LANA staining. Figure C, left panel, shows that LANA+
cells were evident exclusively in the B cell fraction and not in the non-B cells (comprising almost entirely T cells) in agreement with the detection of viral genomes predominantly in the CD19+
(B cell) fraction of peripheral blood cells from seropositive patients (16
). Thus, LANA+
KSHV infection preferentially occurred in B cells within ex vivo suspensions of human tonsillar cells. Of note, in separate experiments, we also directly compared the rates of KSHV infection in purified B cell cultures with cultures from the same tonsil donor (n
= 5) that contained non-B cells (mixed cultures) and found no consistent effect on the overall percentage of KSHV-infected B cells (data not shown).
LANA expression is evident primarily in λ tonsillar B cells.
After subdividing the B cells by their expression of either κ or λ light chain, we found that LANA+ cells were overwhelmingly evident in the λ rather than the κ subset (Figure C), even though each subset was present in the tonsillar cultures in nearly equal proportions (Figure B). The preference for λ over κ was marked, with the latter demonstrating levels and patterns of LANA signal that were barely distinguishable from the background staining evident in uninfected controls. The strong λ B cell preference was evident regardless of the presence or absence of non-B cells in cultures and persisted in cells from two tonsil donors, even at 7–14 dpi within cells cocultured on the CD40L feeder layer.
In contrast to the clear predilection for LANA expression in λ B cells, we have detected PAN RNA in κ B cells and CD3+ T cells as well as in λ B cells, sorted 24 hours after en masse infection of tonsil cell suspensions (our unpublished observations). The presence of this transcript in a broader range of cell types would suggest that, at least under the conditions of our ex vivo infection system, viral entry and pre-latent gene transcription were less discriminatory than LANA expression.
KSHV-driven growth/proliferation in tonsillar B cells.
We observed that the LANA+ cells appeared larger than LANA– cells and hypothesized that KSHV infection may drive growth and/or proliferation of tonsillar B cells. To address this possibility, we compared the cell size (area of the bright-field image) and DNA content (intensity of DNA staining with DAPI) in our cultures at 48–84 hpi. The left panel of Figure A shows a representative scatter plot, with each pixel representing an individual cell, in which we identified and gated cells that were small (<110 μm2) with low DNA content, designating them as resting (R); large (>110 μm2) with low DNA content, designating them as blasting (B); and large with increased DNA content, designating them as dividing (D). The right panel of Figure A depicts representative images from each of these gates, R, B and D. The images show that some cells in the D gate do, in fact, appear to be undergoing early stages of cytokinesis. We then characterized the extent of proliferation of LANA– and LANA+ cells by comparing their distribution within each of the above gates. LANA– cells were predominantly resting (75%), whereas only 10% of LANA+ were resting. Only about 20% of LANA– cells were blasting, in contrast to the majority (70%) of LANA+ cells. Finally, a small fraction (5%) of LANA– cells were dividing, while a significantly greater portion (20%) of LANA+ cells were dividing (Figure B).
Growth and proliferative effects of KSHV on latently infected B cells.
To address more rigorously the link between KSHV infection and promotion of cellular growth, we subdivided LANA+ cells by intracellular viral load, gating them into groups with 1–2, 3–4, or ≥5 LANA dots/cell. We then assessed each group for the percentage of cells that were resting, blasting, or dividing. We found that intracellular viral load positively correlated with the fraction of proliferating cells. The cells with the lowest viral load (0 or 1–2 LANA dots/cell) had the highest fraction of resting cells, while the group with the highest viral load (≥5 LANA dots/cell) had the lowest (Figure C). Significantly, all of the cells with highest viral load, and nearly all of the cells with intermediate viral load were either blasting or dividing. This positive correlation suggested either that KSHV infection drove B cell proliferation or that KSHV more efficiently infected proliferating cells.
To help distinguish between these two possibilities, we compared the proportion of proliferating cells among each subpopulation of tonsillar cells in the presence or absence of KSHV. We reasoned that if KSHV drove cells toward a proliferative state, rather than merely targeting proliferating cells extant in the culture prior to viral exposure, we would find a greater fraction of proliferative cells in the KSHV-exposed cultures than in the unexposed cultures. The results demonstrated that while the distribution of resting, blasting, and dividing cells within both non-B cells and κ B cells was unaffected by the presence of KSHV (data not shown), the proportion of proliferating λ B cells increased markedly with KSHV exposure (Figure D). In the absence of KSHV, approximately 75% of λ B cells were resting, whereas in the presence of KSHV, this fraction was reduced to 45%, while the fraction of blasting cells more than doubled from 19% (–KSHV) to 45% (+KSHV). Likewise, the fraction of dividing cells significantly increased in the presence of KSHV from 4% (–KSHV) to 10% (+KSHV). These findings indicated that KSHV infection specifically drove proliferation of λ B cells.
Notably, the proportion of proliferating cells following KSHV exposure was greater than the proportion that became detectably LANA+
. This suggested that KSHV infection resulted in a bystander effect on other λ B cells. Alternatively, although perhaps less likely, the apparently LANA–
portion of the proliferating λ B cells could be infected but be at an early stage prior to well-organized LANA tethering of the episome(s) to the chromatin. The MIFC algorithm queried punctate LANA dots and thus would not detect cells with LANA expressed at a low level or broadly distributed (46
To confirm that λ B cells proliferated after KSHV infection as suggested by their morphological changes and increased DNA staining (above), we also measured DNA synthesis. We exposed tonsil cultures to increasing amounts of KSHV or, as a negative control, UV-inactivated KSHV, and quantified the number of cells that incorporated the nucleoside analog EdU from 12 hpi to 72 hpi. We observed a clear positive correlation between the proportion of EdU+ λ B cells and the initial MOI. This correlation was absent for both κ B and non-B cells (Figure E). The percentage of λ B cells undergoing DNA synthesis, therefore, increased with infectious dose, again arguing that KSHV infection drove proliferation of λ tonsillar B cells. We also found that UV-inactivated KSHV had no effect on proliferation of any of the tonsillar cells, including the λ cells (Figure E, right panel), suggesting that proliferation required establishment of infection rather than mere viral binding to or entry into the cell.
To further investigate KSHV-driven proliferation in B cells, we assessed expression of Ki67, a nuclear antigen expressed during all stages of the cell cycle except G0
) at 72 hpi. Figure A shows representative images of LANA–
(left) and LANA+
(right) cells exposed to KSHV. A total of 8% of KSHV-naive cells expressed Ki67. Similarly, 8% of KSHV-exposed but uninfected (LANA–
) cells expressed this marker, indicating that there was no bystander effect of KSHV infection on Ki67 expression in LANA–
cells. In contrast, 44% of KSHV-exposed, LANA+
cells were Ki67+
(Figure B). These results corroborated the marked increases we found in cell size, DNA content, and EdU incorporation associated with KSHV infection.
KSHV-induced expression of the proliferation marker Ki67.
We further explored the relationship between KSHV infection and proliferation by grouping LANA+ cells by viral load into cells with 1–2, 3–4, and >5 LANA dots/cell and measured the levels of Ki67. The proportion of infected cells expressing Ki67 rose with increasing intracellular viral loads, reaching nearly 75% for cells with 5 or more LANA dots (Figure C). We next asked whether KSHV infection also increased Ki67 expression on a per-cell basis as a function of viral load. To do this, we calculated the fold increases in Ki67 MFI in LANA+ cells compared with LANA– cells. Again, we found that the Ki67 expression levels correlated with increasing viral load (Figure D), increasing from a baseline set at 1 (0 LANA dots/cell) to 1.2-, 1.5-, and 2.3-fold for cells with 1–2, 2–3, and >5 LANA dots/cell, respectively. We also noted that the maximum Ki67 signal in LANA+ cells exceeded the maxima in KSHV-unexposed cultures. This dose-dependent relationship between viral load and Ki67 expression again suggested that latent (LANA+) KSHV infection was the driving force underlying the proliferation of infected λ B cells we observed.
Evidence of plasmablast differentiation in KSHV-infected λ B cells.
It was of interest to us that the establishment of latent (LANA+
) KSHV infection only within λ B cells was reminiscent of a similar, nearly exclusive λ bias evident in KSHV+
MCD and in at least some KSHV+
PEL B cell tumors (4
). Since MCD is less differentiated and less neoplastic than PEL and, thus, potentially more similar to newly infected, nontransformed B cells, we focused our phenotypic analyses on a comparison between de novo infected tonsillar B cells and MCD plasmablast B cells. Specifically, we assessed LANA+
cells for the expression of surface markers expressed on MCD B cells, namely IgM, CD27, and IL-6R, as well as morphologic characteristics suggestive of the plasmablast stage of B cell differentiation, including cell size and the pattern of Ig expression.
To determine whether de novo infection of tonsillar B cells preferentially occurred within the IgM+ subset, we incubated KSHV-exposed tonsillar cells with antibodies directed against this marker (representative images shown in Figure A) at 72 hpi. We gated tonsillar cells into IgM+ and IgM– subsets (representative gating shown in Figure B, left panel) and assessed these fractions for the presence of LANA+ cells, as we described above for λ and κ B cells as well as non-B cells. Nearly all the LANA+ cells were present in the IgM+ subset while being virtually undetectable in the IgM– subset (comprising both IgM– B cells and non-B cells, Figure B, right panel), similar to the λ bias. This observation narrowed the identity of B cells targeted by KSHV for latent (LANA+) infection to the IgMλ subset.
KSHV-infected B cells resemble MCD plasmablasts.
Furthermore, within the IgM+ subset, the level of IgM expression on LANA+ cells was greater than on LANA– cells (Figure C, left panel, shows a representative histogram) and the fold change in intensity of IgM staining over that of LANA– cells increased with increasing viral load (Figure C, right panel). Together, these findings suggested that in addition to selectively establishing infection in IgM+ B cells, KSHV upregulated IgM expression.
Since we permeabilized the cells before staining, we measured total Ig expression, both surface and cytoplasmic. Examination of individual images revealed two distinct staining patterns that generally segregated with the absence or presence of LANA staining. The LANA–
cells displayed a pattern suggestive of surface (Figure A, left panel) and surface plus cytoplasmic (Figure A, right panel) IgM expression, respectively. The apparent presence of surface IgM on LANA+
cells (as well as λ, Figure A) suggested that they were less differentiated than plasma cells and PEL cells, both of which lack surface Ig (12
). However, LANA+
B cells appeared to additionally contain some cytoplasmic Ig (Figure A and Figure A, right panel) similar to MCD B cells (4
), suggesting that the de novo infected, LANA+
B cells were likely in an early plasmablast differentiation state at 72 hpi.
We next compared the proportion of cells expressing CD27 in LANA+
cells, restricting our analysis to IgM+
B cells, thereby excluding CD27+
tonsillar T cells also present in the cultures. Figure D, left panel, shows a representative histogram illustrating the CD27+
gate as well as the comparison of CD27 expression on LANA–
cells. Overall, CD27 expression was low (relative to T cells) on tonsillar B cells; however, since MIFC allows visual inspection of CD27 expression in each region of the histogram, we were able to determine a threshold gate to distinguish between cells with and without discernible CD27 staining (Figure D, above left). Figure D, right panel, shows that among the IgM+
cells, the latently infected (LANA+
) cells were enriched 4.5- to 30-fold (depending on the individual donor) for CD27 expression at 72 hpi compared with uninfected (LANA–
) B cells. In contrast to our findings with Ki67 and IgM, both the level and likelihood of CD27 expression were approximately the same regardless of the number of LANA dots/cell, indicating that CD27 expression did not correlate with intracellular viral load. Of note however, between 7% and 36% of LANA+
cells were CD27+
, a finding that mirrors the phenotype of KSHV-infected B cells in MCD (48
In addition to the overall increase in cell size (Figure A and Figure E), we noticed that the nuclei of LANA+ cells were larger (Figure E). A hallmark of both normal and MCD plasmablasts is increased nuclear-to-cytoplasmic (N/C) ratio. To assess the N/C ratio of the ex vivo infected tonsillar B cells, we created MIFC software algorithms to calculate the ratio of the area of the DAPI-stained nucleus to the area of the bright field–defined cytoplasm (see Methods). We noted inter-tonsillar variability in the N/C ratio among the 6 tonsils we examined, even though each demonstrated an overall increase in both nuclear and cytoplasmic areas. In 2 of the tonsils, the N/C ratio was essentially unchanged by the presence or absence of LANA dots. In the remaining 4 tonsils, however, we observed a positive correlation between the N/C ratio and intracellular viral load. LANA– cells and LANA+ cells with the lowest viral load (1–2 LANA dots/cell) had similar N/C ratios, whereas cells with intermediate and higher viral loads (3–4 and >5 LANA dots/cell) had N/C ratios that were at least 10% and 30% higher, respectively (Figure F). This positive correlation suggested that, at least for these tonsils, KSHV induced this phenotypic change. In summary, de novo infected B cells shared phenotypic similarities with MCD plasmablasts, including proliferation and expression of Ki67, high levels of IgM, intermediate levels of CD27, and, in most cases, an increased N/C ratio.
KSHV-driven plasmablasts are responsive to IL-6.
Given the central role of IL-6 in KSHV-associated disease (49
) as well as in normal plasmablast growth (51
), we asked whether KSHV-infected tonsillar B cells were capable of responding to this cytokine. We first assessed the expression of IL-6R, a characteristic marker of both normal and MCD plasmablasts (3
), by MIFC at 72 hpi. Figure A shows representative images of IL-6R staining on LANA–
(left) and LANA+
(right) cells. The left panel of Figure B depicts a representative histogram of IL-6R expression on LANA–
cells from one tonsil donor. Quantitative characterization of cells from each (4
) tonsil donor consistently revealed that the percentage of cells expressing IL-6R was similarly low on uninfected (LANA–
) cells, whether KSHV-unexposed (7.1%) or -exposed (7.9%), whereas LANA+
cells were significantly enriched for IL-6R expression (69%) (Figure B). Moreover, the proportion of cells demonstrating detectable IL-6R expression correlated positively with viral load (Figure C). Furthermore, the level of expression on individual IL-6R+
cells similarly paralleled viral load. Figure D shows a positive correlation between fold increase in IL-6R expression cells and increasing viral load, suggesting that KSHV infection led to upregulation of this receptor.
KSHV-infected B cells express high levels of IL-6R and respond to the plasmablast growth factor IL-6.
These results suggested that KSHV might confer enhanced potential reactivity to the proliferative signal, IL-6, on infected IgMλ B cells. This change would be consistent with the potential role of KSHV in driving differentiation of susceptible B cells toward a plasmablast phenotype, since normal human plasmablast differentiation involves IL-6 signaling (51
). To test the functionality of the KSHV-induced increases in IL-6R expression, we added exogenous IL-6 to cultures following KSHV exposure and assessed the cells for growth at 72 hpi as we had earlier (Figure ). IL-6 treatment resulted in a nearly 2-fold increase in the percentage of LANA+
cells that were blasting compared with no treatment (Figure E). We did not detect a significant difference in the percentage of dividing cells in most tonsil cultures at 72 hpi, but we cannot rule out the possibility that division would be similarly affected with different doses of IL-6 or at further time points after infection. These data supported our observations that KSHV induced a plasmablast phenotype in infected B cells and suggested that IL-6 acted as a growth factor for KSHV-infected cells.